Have we done any voltage checks yet? It sure sounds like something is either over or under-powered here. If the motor is new, then the timer sends the current to it from the power cord. As long as you have the correct voltage at the wall outlet, the timer is likely causing this issue. It also controls the Neutral circuit to the motor, not just the hot feed.
If the timer is telling the motor to run at 2 speeds at once, this can occur also.