You can try using MGT servo savers with the JR top piece. The MGT ones use more springs or stiffer springs.
Although, it sounds like you may have water or mud in the tires throwing them out of balance. Or, your foam has torn and gotten bunched up in one spot.
If it's water, usually you can drill a couple holes in the center of the tire itself at 12 and 6 o'clock. Then when you drive it at higher speeds, centrifugal force will spin the water out of the holes.
I put holes in all my tires to avoid water building up in them from snow/wet running. Works really well. I put a smaller than 1/8" bit in my dremel and gently apply pressure to the tire as the dremel is running. This way, it burns a clean hole through the rubber to help avoid tearing at that spot. I have never had a tire blow apart because of these holes.
