welp, i was in the same situation as you
playing the game for some minutes, winning some races, and then the game suddenly starts to drop to 2-8fps, after some time it just crashes
I solved this by lowering the ammount of cpus wich the game can run with
its pretty simple, open your game, minimize it
open task manager, right click the game .exe
click in "go to details"
set affinity
unmark every cpu except CPU 0
well, this will help you if you are using a multi-core CPU
this game was made for way slower cpus, so it can get confused and frustrated when you use multi-core cpus
also, make sure to create a new game after you reinstalled the mod with the Megapack and the Patch
this definetely fixed the thing for me
i'm having many hours of joy playing this game now