Misfire?

[TJPittbull]

Occasionally under heavy-ish acceleration, the check engine light will flash, indicating a misfire. The first time it happened, the light stayed on continuously. I called OnStar, who told me that the check engine light was on because a misfire was detected. I took it to my dealership, who reset the code, and could not find anything mechanically wrong with the car. I drove it home, and it was fine. The check engine light will flash occasionally when under acceleration, which according to 5-18 (or about there) in the owner's manual, is stated to warn of a misfire. I called OnStar again, who told me to try different gas. I have emptied and changed the gas since the first occurrence from a different gas station. Any ideas what might be the problem?