My FPS is acting weird

Hello, my FPS rate is acting strange. For example, when i spawn in on Gridmap, the fps is around 90-100. But then when i start to drive around, its goes down to 50, then back up to 80, then goes down to 30, and keeps going back and forth. Does anyone know why? My specs are:
Nvidia 960M, Intel i7-4720, windows 10 and 15.9 GB of RAM. Thanks.
Edit: I tried in safe mode, and it didnt fluctuate back and forth, so ill find out which mod it is thats causing the issue.
Edit: The mods dont actually have an effect on it. I tried the game in safe mode some more, and it happens, but not so much anymore.

 

does it do the same on other game?

 

No, it only happens on beamng.drive

 

Can we maybe have a video with fps counter? (That can drop some frames)

 

Okay, so now the issue is still there, but it only occurred once. And i discovered that in windowed mode, the fps drops about 12-15 fps, but when i get back in full screen, it climbs back up, but its about 5 fps below normal. This is what its normally like on gridmap.
http://media-cdn.beamng.com/2qRKHOgvegSPXROz/screenshot_00001.jpg

My fps was fluctuating a little, but thats it. On west coast however, the fps was really weird. Heres a video:


It was around 45-50 fps, but then it went down to 35 fps momentarily.
Heres the banana thing:

 

yeah ill inform you when this occurs again, thanks for your help!

 

Is your pc physically dusty?

 

A little, but its only dust that accumulates over time. Does it affect the performance?

 

Well if it accumulates enough it can block and choke fans, and could overheat components, shortening their life span and can affect performance but a little likely won't do anything.

 

Yeah, it only covers the top of my laptop, and i just swipe the dust off once in a while