Strange Lag Spikes

Until yesterday, BeamNG has run flawlessly on my machine. Today, I began experiencing random occurrences of lag, where the game pauses for a half second and the sound stutters. It happens whether physics is paused or unpaused, and happens regardless of settings changes. I routinely run on 1280x720 all settings maxed at 30-50fps, but this weird lag happens even on lowest now. Using the K+L Debug screens, I've noticed that on the graph in the lower right, the red line labeled "dt" shows a massive spike whenever the lag occurs. What does this mean and how can I fix it?

Specs:
AMD FX-4100 CPU oc'd to 4.4GHz
8GB DDR3 1600 RAM
ASRock 970Extreme3 Mobo
4x500GB HD
EVGA GTX 560SE GPU oc'd to 875/1950/1750

My CPU stays under 50C, GPU under 65C, and Northbridge under 70C so I know I'm not thermal throttling, is there a cache somewhere for the game that I can clean out? Are there any issues with large amounts of installed custom content? I have probably 25 vehicles and 10-15 maps installed. I've been playing with jbeams lately, modifying them and making custom parts for vehicles I've downloaded. If I can fix this lag issue I will be making my first custom vehicle soon but it's barely enjoyable and incredibly frustrating to test things out with this lag.

 

I find that the menu screens at the start of the game are really slow with lots of content installed, but as far as i can tell it doesn't effect in game.

Have you checked your CPU usage while running the game, is it possible you could be doing other cpu intensive things at the same time as playing?
Using the debug screens check what fps the physics simulation is running at as well as the graphics. If the physics is running slow then that could indicate your CPU is suffering. I say this because i am using a 560ti which is similar to your card and i can get 50-60fps at 1080p, this makes me think your cpu is acting as a bottleneck because it cant handle the physics simulation.

 

During the game my CPU never hits 100% usage, it's usually between 50-60%. When these stutters happen, sometimes it's at 100% on a single core, usually core 1 or 3 but not always. I think I solved the issue though, I cleaned out my game files by copying my levels and vehicles folders to a second hard drive, reinstalling the game and picking through the backup folders to reinstall only downloaded content that I know for sure works. That was this morning and so far today, 60fps@720p.

I know this game is CPU intensive but the FX series are beefy CPUs and I'm running a pretty crazy overclock and in every other game I play, my 560SE is definitely the limiting factor. I'm considering an upgrade to an R7 260 soon.