Since it's so intermittent, it might be better to check for a voltage drop. The 120 volts that's suppose to be going to the motor is dropping out somewhere. Set the timer like your starting a load, timed dry is good. Check for voltage between the black and blue wires at the timer while pressing the start button. If you get 120 vac, that's where your voltage is dropping out and your timer is bad. Do the same test at the thermal fuse. Of course, that means you're working with live voltage, so be careful. If you're not comfortable with that, unplug the dryer and do a simple continuity test. Same thing, timer set to run. There's no need to disconnect the wires unless you want to confirm the component, just disconnect one side.