They are probably out of balance -- just as with full size car wheels, it can pay to balance sedan tires, expecially in Mod classes...
I use a Top Fliight "propeller" balancer... Go to a Golf shop and get some of the thin lead tape they sell for balancing golf clubs, and use it on the inside of the rim as "wheel weights" -- It's self sticking and soft, so if you put on too much, just slice a little off with an xacto knife... Easy...
This assumes that your axles are spinning on good bearings and not worn out ones or bushings....
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad Track: The RC Race Barn
TLR22-3.0, TLR22T-2.0, TLR22SCT-1.0