If you accellerate slowly going down the highway can you slowly gain speed up to 70MPH? Misfire while trying to accellerate quickly is usually a problem with the MAF or MASS or MAP sensors. If you cannot go up to a higher speed under heavy or normal accelleration, you also can have a fuel delivery problem, low fuel pressure, weak fuel pump, clogged fuel filter, problem with clogged intectors, etc. The best thing to do is have the computer scanned for the data and watch live to see what is happening under heavy accelleration even if you are in the parking lot.