FPS drops the longer i play

i have this problem in beamng that after ive been playing for like... 20 minutes, with the same car on the same map, the fps has dropped pretty much. when i start i have around 40-50fps but when ive been playing for about 20 minutes it has gone down to around 30, sometimes it even goes down and locks at 25 after playing for an even longer time.
ive also seen that it only uses 4 of my 8 cores and my cpu isnt working very much, like 30% when playing beamng.
specs:
AMD FX-8320 8-core @4.0Ghz
MSI GTX 770 2GB
8GB 1600Mhz DDR3 RAM
700w/750w powersupply
Windows 7

 

It could be overheating. Have you checked the temps while playing?. Also low cpu usage is completely normal, you say you're playing with only 1 car.

 

overheating is not a porblem, got a freezer 13. temps are around 20-30 when playing beamng

 

20-30 ºC? Your ambient temp must be close to 0ºC then... What about GPU temps?

 

idk but its a GTX 770 on stock clock and it has two fans so i dont think that would be a problem.

 

One more thing. You say you're only getting 40-50 fps? A GTX 770 and a 1440x900 resolution should be enough for +70 fps on highest settings.

 

ik thats a problem i have too.
EDIT: i think somethings wrong with my cpu, i currently cant get above 35 fps

 

I would of said that it is overheating and the CPU is throttling.
Also where are you getting the CPU temp's from.

 

core temp, but im pretty sure its not overheating, freezer 13 cpu cooler....

 

I wouldn't trust that program as far as a could throw it, it said that my CPU idles at 9C. Does your motherboard have a tuning/overclocking tool/program that will tell you the temperature as that seems to be more accurate.
Just because you have a big heat sink doesn't mean that your CPU wont overheat as you might not have enough airflow/ventilation in your case.

 

no i didnt get any that will tell me the temperature except for in the bios, but i cant enter bios while in a game

 

ignore this..

 

I don't have a 770. I have a 7950 boost, and I just guessed that if I can get +70 fps then someone with a 770 can too. Oh, and 1440x900 definition. Maybe you're playing at 1080p.

 

im getting these fps results with lower that 1440x900, 1280x1024 i think

 

Actually 1280x1024 has 14720 pixels more than 1440x900, though the difference shouldn't be noticeable in terms of fps.

 

could be power supply....... ... ...?

 

being to weak? its 700-750w

 

Can you check cpu clock while playing? Let's see if it's thottling or not.

 

what brand? im kind of going out oon a limb but still.... never know...

 

just check in windows task manager when playing?
how do i see if its throttling

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FSP700-50ARN

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(imported from here)
its in swedish but you will understand anyways. this is when running beamng