Sounds real similar to the problem that I was having, on the opener of coarse . The problem with mine was the foot pedal. After dissassembling the foot pedal I found that it consists of 5 contact points. The one that was causing me problems was the one for the variable speed selector. Its simply wasn’t pushing down hard enough and telling my motor how fast to go. When that happens, it would just click on and off rapidly. So what I did was put SMALL gob of 2 part plastic epoxy to increase the length of the tab that pushed on the contact. Solved my problem.
If your motor is still under warranty I would definetly take the motor or the foot pedal in to have it looked at first.
The next step would be to get a cable drive trolling motor . I know thats my next step .