Been doing some reading on continuous current of the focbox, looks like they are almost certainly overheating with the 60A motor current. So I will need to add a chunk of aluminium to the bottom of the one for the drum and change the thermal pad for something a bit less rubbish.

As for the bad drive reliability, I have ordered some MOD1 spur gears, 12T and 80T, which still lets it go 19mph, but with almost twice the torque. Going to increase the wheel size from 70mm to 85mm so it gets stuck on the floor less. I am fed up trying to get belts to work right, they are very finicky and an easy target.