FPS decreasing within a lap

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Hello community,

This is my first topic here because I would like to get some help from you, as I don't understand what is happening.
I used to play with FPS limit at 60, running without any issue.

So, when I start a track ( hotlap or race ), my fps decrease during a lap, from 80 to 30 and when i cross the finish line, it jump back directly to 80 fps.

I tried many scenario : Hotlap / race / with 8 racers or 20 / with stock car or modded one / with or without FPS limit on non modded track. I also tried "steam file checking".

I add issue last week as i tried to install Sol mod but it didn't work, so i uninstall the game a intall it again. I think it didn't erase properlly all the file. I don't really want to do it again as i spent time to install mod etc, but if there's no other solution, I would like to get the exact way to do it properlly.

I also check log file and i see issue but I don't know how to fix them.

Could you give me some help please ?

Thank you,