It may not be your problem, but I had a similar problem in the past with a metal-geared servo in my Tamiya TA-05.
In that case it was caused by static electricity building up in the plastic chassis and discharging through the servo. Tamiya even acknowledged the problem and KO came up with a solution involving a jump lead linking the motor mount to the front bulkhead.
If you can see an opportunity for static to build up in your cars (non-conductive materials rubbing against each other in motion), perhaps you can find a similar solution.
The static issue did not affect the TA-05 if a plastic gear servo was used, so perhaps try one of those.
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