2003 Astro intermittent HVAC problem

Hi everyone, there seems to be a lot on this subject, but I can't seem to narrow down my particular problem and was hoping to find the solution in this forum. I have a problem that only occurs sometimes and it's with the heater blend door flapping around back and forth. Most times is works just fine and other times it does not. It mostly flaps around at higher fan speeds. I replaced the vacuum tube from the intake to the HVAC T-valve hoping it would solve the problem but did not. Some people said its the heater blend door actuator that needs replacing, but I thought I would check here first before I start replacing parts. This also occurs sometimes when the vehicle hits an above average bump on the road. Any help in this regard would be much appreciated.