Pontiac Repair: Bucking in a Pontiac Montana sv6 2007, pontiac montana sv6, high performance cars

Question
A few months ago, I went to a local shop in my city and got an transmission and oil change done.  A few days later I noticed that at times when starting to accelerate we get a slight pause or a bucking motion in the van then we get going(it seems as though someone had hit us) but its just us getting going.  This doesn't happen each and everytime but has been increasing!  what do you think is our problem....we have taken it back to the shop numours times and each time the mechanic takes the van out for a drive it doesn't happen.  I have had many different things changed such as ball joints and sway bars in the vehicle and still the strange thing keeps happening?

Please help me...help the mechanic to figure out our problem!

Answer
Hi Jennifer:
Has any service engine soon lamp come on ? Does it slam into gear once it does "catch" does it happen hot or cold. any kind of slipping in a transmission is not a good thing the slipping wear out parts faster then then would on their own. My giess is possibly a oil pump problem in the transmission or a pressure control solenoid in the transmission is acting up. Just a guess. Another though would be if the wrong transmission oil was put in this it could cause a wier trasnmission shift. all of the GM fluid is pretty much the same untill you get into the high performance cars BUT if Ford trasnmission oil was used instead of the GM Dextron it can cause shifting issues. The fluid properties are different between the 2 fluids. Especially if this started to happen right after the fluid was changed. It's also possible that when the fluid was changed or flushed some dirt may have wound up in one of the valves in the trasnmission control body Not that uncommon for that to happen. But then again that is a guess as well. good luck :-}