At least twice in my last 10 flights, the engine misbehaved upon run-up. As I advanced the throttle, it misfired, meaning there was a definite ?bang? (not a back fire) or two on the way to 4000 RPM. If I left the at 4000 RPM for a few minutes, the problem did not reoccur upon a second run-up. All temps were low, both engine and OAT. The gas was about a week out of the pump. I do have a longish taxi from the hanger to the run-up area. Fouled plugs? Again, the engine did not ?stumble?, it was a bang or impulse for lack of any other way to describe it. So, .....Cause? Avoidance? Negative effects?