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

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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...