A/C and heat works but sometimes with force
 

I am an owner of a 2005 Vibe AWD. About a year and a halfago i started haveing problems with the air system while i was in alaska the cabin filter became clogged and restricted air flow ao i removed the filter and air flow was restored. About six months after that while on a drive from alabama to north carolina my air system decided is was going to have a mind of its own. Every now and then i have to smack the bottom of the blower motor houseing to get it working after that it works but some time it lowers air volume output when the brake is applied or at sustained speeds for long periods of time. Has anyone else come across this problem and if so what would be the best course of action to take short of replacement of the blower motor itself. Checked all the fuses i could find related to the system and all seems to be ok gecause its still working just gets temermental from time to time.