Swan,
Do you have a seat switch on your cart?
Just a hunch, but if your seat switch is malfunctioning, going over the speed bump might shut down your controller ... perhaps a long shot, but worth checking out. I once had a riding mower with a bad seat switch that resulted in a similar problem that started out intermittent and then completely failed. You can disconnect and jumper your seat switch connector as a quick check.
Otherwise, figuring out if your solenoid is disengaging during the failure should help your troubleshooting as, WalterM6 was alluding to.
You might also want to make sure your key switch is functioning correctly as it too could be responsible for this problem.
Just some "brain storming" ideas here ... either that or "brain farts" ... not sure which!
