I have had this problem multiple times. Most often it is height switch related. Have you followed the height switch cable all the way down to the connector, unplug it and use a paper clip to jumper it? I know you read power at dist board but I had one that had a bad spot in the cable and it was in the area where the cable would move when the mast was tilted. It would read good if the mast was in a spot where the cable was ok.

I also had one with a bad steer pot and the controller would slow down the motor speeds as well as several bad accel pots.