There's only one thing I can think of. Are you using the top screws in the motor plate? We were running without them for a while so that we'd get uniform flex across the top deck on the old car and noticed that we were going through spur gears like nothing. The motor plate and lay-shaft mounts being separate pieces, if the motor plate is not screwed into the top deck, the chassis flex will make it move independently from the lay-shaft, changing the gear mesh. I had a 64 pitch RW gear strip going through a high speed sweeper at SoCal once because of this. Even if the screws are not good and tight, the flex can cause this to happen.
As for what brand of spur gears, I like the RW gears and the PRS gears. If you're using anything other than the stock gear, make sure you notch the pulley where the screws go so that you can mount the screws in the inner holes on the adapter.