installed? Did you changed anything just prior to the onset of the problem? Have you checked for bad connections and bad grounds? Is the wire size large enough to eliminate the voltage drop issue? Does the problem occur only while certain other pieces of equipment are turned on?
I would look for voltage drop or high resistance first if the unit worked for a season or two and then started showing the problem. I would jumper the switch to eliminate it as a source of the problem. I would put a meter at the motor or as close a possible and check the voltage. I would check the fuse for corrosion. Good luck.