Sounds like the springs may be too soft.
Gearing may be too high but it is difficult to comment without knowing the track.
Check the transmission is free.
If you are skimming the motor yourself get someone to check the finnish on the comm before you use the motor. Skimming the motor with a blunt bit or the bit set incorrectly will give a poor finnish on the comm which will have the effects you describe.
If you are using comm drops this may be the problem. Some are designed for break in only. Also if you use comm drops you must use them sparingly and clean the motor after every race or dirt will accumulate and cause premature wear.