Pontiac Repair: Hesitation after water pump replacement, water pump replacement, loose connections

Question
I recently bought a used 96 Grand Am SE 2.4L (52,000 miles)that needed a water pump replacement.  (have to go into the engine).  After I got it back I immediately noticed that I had hesitation and bucking on acceleration when first starting and the engine is cold. (Doesn't hesitate after warming)  Took it back to the mechanic who checked for codes (none) and basically blew me off.  Something wasn't done right for me to have this new symptom after the internal work.  I need an idea of what could be wrong.

Answer
well first off it sounds like 1 it might have a damaged temp sensor, or maybe the vacume leak on the engine, it could be several things , i would suggest opening the hood and look for loose connections or vacume lines by the waterpump area. if a qualtiy shop did the repair they should check thier work for you and help you