2005 STS acceleration "bucking"
 

I have a 2005 STS with the 3.6. I have a constant problem that when accelerating the car will lurch or buck like a really bad misfire is happening. Usually no lights flash. I have even been driving down the street and Onstar has called me asking if everything was ok?! I couldn't tell if anything was wrong but they said that the engine was misfiring...no idiot lights. I took the car to the dealer and they replaced the coil and it seemed to fix the problem. But, then it came back, probably worse. No codes. Today it bucked so violently that the check engine light blinked then stayed on solid. I called Onstar but by the time they were able to do a diagnostic the light was off and the car was running fine. They said they couldn't see any codes and to call back when the light was on. HELP please.:confused: