My front blower for the fan/ac/defrost works intermittently. What could this be? on 2007 GMC Acadia

I have had it in the shop when it isnt working and they have said that it was the blower motor getting ready to go out. However, When I brought it in it was working and they said that they could not find anything wrong with it. Could it be something other than the blower motor? A short in the wiring some where? When it does work the fan speeds all are normal. It seems to not work when its cold out, and as it warms it up outside, or maybe hit a bump, turn a corner, etc.. It will come on instantly...

Asked by for the 2007 GMC Acadia
could be blower motor going out of possibly the blower motor resister is bad