[Doug Tatham]

Final update:
It was the fan. You can't find an aftermarket one that fits all that well, but you can adapt the ones that they sell with two wire leads fairly easily.
Pull the plug off the old motor, solder the leads to the backside of the plug and epoxy the plug to the edge of the shroud. The holes in the shroud for the fan will have to be drilled out a tiny bit to accommodate the screws that they provide.