Unplayable 0.16 with once-per-second hitching

A consistent pause happens once per second on both of my computers.
1st machine:
Radeon 7850 2gb - Drivers from April of this year.
4790k
32gb ddr3 dual channel cas-11 2400mhz
SSD
Windows 7 Pro

2nd machine:
Radeon RX 480 8gb - brand new drivers whatever is current
Ryzen 3700x
32gb ddr4 dual channel cas-15 3000mhz
NVME SSD
Windows 10 Enterprise (evaluation version for 90 days).

BOTH machines hitch once a second, the 4790k is a complete slideshow, where it's stuck for one half of a second and goes for the other half, the 3700x hitches for about 50~150 milliseconds every second (it's much better, but it still does it).
This error happens in my Los Injurus city map, in Nevada Interstate, also in Bajarama as reported by some users in the review.

It's to the point where if it wasn't for Patreon backers, I'd can my city project outright and just forget about it. However, I can't do that to those who have donated money to the project, and I'd like to enjoy my maps with the new vehicles sounds and whatever other features (besides major lag) that have been added.

What goes 150~180FPS on my map on 0.15.x game version is <55fps and stuttery unplayable mess on 0.16.x game version.

The only thing helps is playing with shadows off. Sure that fixes most of it, but seriously, 2019, brand new machine and can't have shadows?
Again, tested with only my map mod and the default truck.

 

Not sure if this has anything to do with it but I do have your Nevada Interstate map and my FPS takes a massive hit anywhere streetlights or lit tunnels are visible from. Everywhere else it's good. GTX1060-6, Ryzen 2600X for reference.

 

@Nadeox1 I can't get it to act up on the stock maps, that's what has been the main issue here.
It's only doing this on my LOS INJURUS City map, Nevada Interstate, and then some reports of it happening to others on your Bajarama conversion.
However, here's the weird thing about it. I loaded West Coast USA on medium graphics. Then I loaded this map, and switched it to LOW graphics from medium (as it doesn't run well here on this 7850 radeon card). Kept shadows on the whole time. It was FINE for the first time in months - and then a minute later when I clicked around to rotate the camera, and moved the performance graph, then it went batty again with the lag, which you can see on the graph. Maybe it's something to do with the mouse cursor or something of that effect? Right after '90' in the graph, it starts lagging.

It's something to do with the shadows, as without them, those big SHADOWMAPPASS (shadow map pass, pale pink color, bottom graph) spikes DO NOT appear. Something changed between 0.15 and 0.16 that just totally chunked performance on my end in several maps. There's several other users this is happening to. Oddly, it's no-where near as severe on the Ryzen, but it still causes a hitch leaving the game impossible to play, though the hitch is MUCH less pronounced.
Willing to try a several-year-old video driver here to see if it helps, though it might bring the spiking / glitching back with the ground cover.

*When I installed Windows 10 on the new machine, it's all new hardware except the AMD card. I installed only what was needed to have basic functionality. Not even the web browser or virus scanner was on at that point. First thing I did was test 0.15.x, which works well (and blows this intel machine out of the water).
Then I tested 0.16.x, which gets 55fps done on the ryzen (compared to 10~12fps here on the intel) and then skips (the skip here is much more pronounced on the intel computer).
Thanks for not putting copy-protection on this, it enables me to do performance tests on a second PC. I don't have any nvidia cards in the house except some built-in geforce 8100 video on the email computer - but that is from 2009 or so, so it obviously wouldn't even go 0.5fps on gridmap - and I'd probably blow it up in the process.

Basically, the only way I am going to fix this is to buy an nvidia card?

 

If the lag does not appear at any official map, then this is mod related. The modder is yet to optimize their mods to the 0.16 version.

 

It's my mod. Also does it on one of Nadeox1's mods that he converted to the new game version.
@Nadeox1 this is a screencap from the Ryzen PC
--- Post updated ---
edit:
PNG file for skybox = lag-fest

Deleting them made a complete 1fps (yes, somehow worse) slide-show with spamming 'can't find file for skybox etc' in the log. So I threw in the ones from west-coast as they shared the same naming.
Bang, smooth fps.
What is weird, is the West coast has .dds and .png sky box files, but it doesn't lag like that in west coast (the .png files aren't referenced in this test, so likely not used, but the .dds files are which are faster).
Attachment is from previous post, ignore.

 

Those are the cubemaps. They're identical to the ones that caused issue. The ones in Los Injurus City Map were originally from Black Hills / Nevada Interstate. All those maps had bad lag in 0.16.
Switching to the .dds ones you guys used in West Coast solved the problem (the materials.cs in these maps point to files without extension, so it doesn't need modification when you switch files).
I had known about switching to .dds for a while, as it's faster, but did NOT realize they were still in there.

 

Yes, it mostly affected AMD users - however - one exception to that report is the following:
Intel 7980XE processor, x299 motherboard, quad channel memory (3200mhz if memory serves correct), and a Gefarce 1080 Ti card on Windows 10 Pro (current version).
Using Nevada Interstate (also Black Hills), when viewing in free camera, turning the camera so that different sky box sections (png files!) come into view causes a little skip every now and then (not as bad as constant hitching, only in free cam when turning around). This is actually CrashBoomPunk's issue with Nevada Interstate, I've already told him to delete the skybox and put West Coast's skybox in (it references the same files, West Coast's are DDS files), which fixes the issue.
Thank-you for looking into that and I am glad that it's finally caught. Took me 6~8 weeks to finally find it (!!!), but I did. This was one of the trickiest ones to diagnose.
What really puzzled me is that it DIDN'T always do it, 98% of the time it would but the other 2% of the time it ran like there was never an issue. Just chalk it up to being pre-release software.
Oh the fun & follies of Early Access, where else could have you this much "fun" ?
--Cheers!