Difficult problem to diagnose when it is intermittent.
Possible suspects are the start switch, motor switch and the motor. I would start by testing the start switch. You can do this by unplugging the machine, taping together the leads on the start switch and plugging the machine back in. If it starts (and do this enough times to test it) replace the start switch and see if this cures the problem.
If this is not the problem examine the motor switch for loose contacts. If you want you can replace it but it is on speculation.
Good luck, let us know what you find out.