its the cpu, i have the same prob for a couple of days since they r moving servers or so, normal fps 75-80
when crowded, like 32 racers 25 fps
i'm buying myself a new sweet rig
for your problem, if its a rather old pc, renew the cooling pasta(could do alot of help)
check the temperature...
update drivers,
and when playing lfs, turn of msn and all the background stuff you dont need