Ah the push button switches are cr@p!
Odd how it still happens after replacing the switch though. Checked the contacts under the cord housing? They could be nearly snapped, causing the symptoms. Cheap to buy, easy to fit.
Belt and braces, is to wire the motor straight to a mains cable using the appropriate crimps, switch it on at the wall, check it runs sweet. This will rule the motor out
Not seen one in much detail, but their the usual Henry faultfinding tips! It could also be a PCB