Recently, I've had a problem with my fan blower when I first turn the a/c on. When it's hot and I'm first cooling the car down with the fan speed switch on high, the blower motor will fluctuate somewhat in speed like it's wimping out down to speed 3 then back up to speed 4. The other day, it did this and I turned the switch to zero then back up to speed 4 and nothing happend at all. It stayed as if it were on zero, so I thought the motor had died on the spot. When I got home, I pulled the front console back to expose the speed switch and the passenger side panel exposing the side of the blower motor. I then did a "Fonz" and smacked the side of the blower motor housing and it instantly came to life at full speed and the speed selector worked perfectly through all the settings. Yesterday, it died again, so I took the side panel off and again smackedd the side of the blower motor and it again came back to life. What could be the problem here? I really don't look forward to replacing the motor if I can help it. Any advice would be appreciated.

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'86 sedan with 345K miles with original engine and automatic transmission