Hvac blower

The hvac blower on my 2000 malibu was not working at all until i gave the underside of the dash a smack. Now it works on high speed only, but gradually winds down to a stop within 30 seonds or so. If I turn the fan switch completely off then on again it does the same thing. Any ideas? Could it be the relay? the resistor was replaced about a year ago -could it be bad?
Thanks

A year or so ago I had a similar problem. It was not the resistor card, it was the connector at the blower motor. You can test yours by moving the connection from side to side. I purchased an after market blower but the wiring connections were different. Big problem in such a tight space, not much room for the fingers. So I returned the aftermarket one, purchased a genuine GM blower, installed it, clipped the connectors in place and all was right with the world.