Sorry to hear...it may have started at the switch since you've replaced the brushes, etc... Was anything wrong with the brushes or commutator when you opened it up...?
Check the operation of the switch with an ohmeter and if you're comfortable troubleshoot the switch under power. Check solder points to make sure they're not loose...
Check the operation of the switch with an ohmeter and if you're comfortable troubleshoot the switch under power. Check solder points to make sure they're not loose...