If works fine first but start acting up after a while, it's either a bearing that's acting up when heated, or some component that's expanding when heated and binds.
As suggested, I'd:
- replace bearings at least on the rear drivetrain ( I suspect the 15x21x4)... dont try to fix bearings, chuck them in the bin and get new ones. If you can afford 8th scale then you can afford $15 of new bearings
- Make sure the rear diff is shimmed with *some* lateral play
Good luck - intermittent failures are a total PITA
Paul