you say the motors are "all right" but you should give us information on how you know the motors are all right. what tests, and what were the readings and how were they tested, as specific as possible.
If this is a dual drive motor setup, I would also confirm the steering is being properly seen in both controllers.
I would still have the motors rebuilt by someone that rebuilds a lot of forklift traction motors (not someone that only rebuilds really little DC motors [like a starter shop] or big A/C motors mainly), if new or rebuilt controllers are failing.
Some of those Curtis controllers can 'feel' some things in the motors a meter with a weak battery might not pick up upon, that would make them fault out.