Like the title says, the car that i have 98max, 5spd has this thing where some times when you stop at like a light or stop sign etc the car hits ~600rpms then they drop to about ~400 for a litle while(split second or so) then it rise back again. Almost sounds as if the car is about to stall but it has only switched off about twice so far. Any suggestions on this.

Another question is that my friend gave me a set of cheap rims coz mine cracked(dont know how) but he had 205/40/17 and first they look stupid on the car, too small. I did some calculations and the conclusion is to stay withing 3% of the odometer speed i can either go 225/45/17 or 215/50/17. I dont autocross but i do take corners fast once in a while but very rare. the rims are 17x7. any input welcome.

Thx