OK, made a discovery today, that might help some people. It was driving me crazy since i bought the first URD GTE car (modern one), and was getting me even more crazy with gt4 pack from Guerilla. It also happened on other mod packs here and there, but not so often. And all that even though i had normal stable FPS rate. I kept getting this micro stutters of picture, like frame drops. Sometimes every few seconds, sometimes not so often. I was trying everything, thinking it had something to do with video settings of my PC. I gave up eventually, didn't drive gt4s at all even though i like them a lot, and I was always struggling with GTEs. Couldn't give that up, as Aston Martin GTE 2019 from URD is in my opinion the best car to drive in all sim racing.
Today i was going through settings and opened the audio_engine.ini file. I remembered that it was a thing a while back to change the number of audio channels from 256 to 512. Don't remember why. Had something to do wit ACFan's and Fonsecker mods not working correctly. And i had this setting again on 256. Don't remember putting it back. It must of happened when doing an integrity check after a power outage and my AC wouldn't start. Then i started thinking, both these packs have very loud intensive sounds of cars, i have an ACFan mod on some GTEs....
Anyway to shorten the story, changed the channels from 256 to 512, and no more stuttering of picture in both mod packs. IT WAS THE SOUND!!! THE ENTIRE TIME, IT WAS THE SOUND!!!!
If you have similar problems, the file audio_engine.ini is located in: ...\steam\steamapps\common\assettocorsa\system\cfg
Change the value of max channels to 512. It should then look like this:
[SETTINGS]
ENABLE_AUDIO=1
MAX_CHANNELS=512
UPDATE_INTERVAL=0
ENABLE_PERFORMANCE_SPEW=0
LIVE_UPDATE=0
This doesn't lower your FPS and you won't brake anything in AC by changing this and you can always change it back. The hours I spent fixing this, AAAARRRRGGGGHHHHH!!!! It makes me so mad.