Shift points

I have a 1995 mustang GT with a 5.0 and 4 speed auto. I have just recently had it rebuilt with a new converter, rebuild kit, shift improver kit, etc. Etc. 1600.00 dollars worth. The shop was a reputable one but I have had problems with shift points before and after the rebuild. The car (especially when you turn) wants to shift up. In some instances the car is in third before 25 mph! The shop even said it seemed early but could not figure out what was the problem? I had a new speed sensor put on it previously and it seemed to help but it is still just shifting wayyyyy early for me. It makes the car seem mushy in the throttle and I think it has to putting strain on the engine as well. Should I just bring it back and say that I am unhappy? Is there a reason for this that anyone can guess? I am clueless on auto's, well transmissions in general!

Help!