Question for the experts: is it possible that the variable speed control, or the board, is bad?
From my experience with machine tools, a faulty ground (ie. Loose ground wire, broken solder connection, etc) can cause all kinds of wierd problems.
If the mechanism checks out - no loose joints, well lubed, etc, - then it may be time to start looking for a short somewhere or a bad ground wire. Just a thot.
So far, the motor has been changed and a connecting rod assembly, correct?
If the varible speed control had excessive dust in it, would it do this?