Sounds like the relay is not kicking it up into high. The fan operates on two speeds, high and low...high should kick in when the gage temp. gets above 210 deg. or when you turn on the A/C.
It also could be the motor...but as you noted, that's not typical, they usually work or don't work.
Have them check it out where you had the work done... they can jump the relay and see how it acts.