Hmmm. A cold solder joint or plug connection seems possible. Bad, weak roller wire, dirty roller INSIDE the axle hole, etc.
Are the lights dropping out? Id think it was a pre board wire issue if so.
Check it, top off, see how hot the motor(s) get. (Careful, or you might be sucking on a finger while headed for the burn cream )
Check powed, in Neu., and see if you can help it drop out by wiggling motor tabs, plugs to board, etc.
If I couldn't find a physical reason, I'd reflow ever solder joint to the board. Maybe on the board if it isnt too full of micro components (faster and easier than it sounds... and I just hate micro because it is so small, really the same deal. I never looked at mine to know)
After that, I'd look to the triac/transistors that do the switching (follow motor leads traces to likely (two?) sub-circuits, likely one for each direction.)