Before you look all over for bearings, which may or may not be bad...Take the wheel/tire that you just removed and shake it around...If there is dirt rolling around in there, there is your balance problem. I have found that the breather holes in the rims let in some grit, and don't let it back out, and after some time it just gets worse and worse...Plus the grit in there is slowly eating away at the foams in the tires.
SOLUTION...
Drill small holes in the tires between tread lugs as close to the center line of the tire as possible. I usually drill 2 holes in my tires. 1/8 inch drill bit in dremel tool works good.

Once your wheels are empty of excess material, or when you put together new rims/tires/foams, you can set about correcting the imbalance that probably still exists. Just like 1:1 scale wheels and tires, to get the best performance out of them, they have to be balanced.
I don't have a fancy balance stand. I just disconnect the drive line from the axle, and let the wheel spin free.(now is the time to service those wheel bearings, if they need it.)
Gently spin the tire, and mark the top of the tire when it stops. Do this a few times to insure the wheel spins freely, and the heavy spot settles to the bottom.
Now glue pennies, brass bushings, nuts, bolts, fishing weights, or whatever else you can grab to weight the inside of the rim.
I usually just use brass bushings from old projects(see signature[sm=50_50.gif]), and glue them with tire glue.

There!! Now your wheels don't wobble, and your acceleration has improved, and your top speed is higher, and it handles like it's on rails...NOTHING can stop you now!!!(except that parked car...[:@])