While driving on the freeway last night (70-75MPH), out of the blue, I discovered a rapid stutter upon rapid acceleration. My car had been running perfectly smooth, even when speeding up slightly. But now,
when I was speeding up a bit more aggressively to pass, the misfiring started to occur then vanished upon easing up on the accelerator. At first, I saw no engine light when this happened, so I forced the issue and was able to get the engine light to flash on and off.
Today I hooked up my OBDII and found no code. Next, I drove the car with it hooked up and it showed the misfires in #4 (12-28 count) depending on which time I reset and read the summary. Lastly, I once again forced the issue and prolonged the stuttering by accelerating harder for a longer period and this caused the engine light to stay on and I was able to get a code P0304. I swapped plugs between #3 (good) & #4 (bad) cylinders, then later swapped coils between 3 and 4, each test found the misfire remaining in #4 cylinder... It drives great through all gears until I get on it just a bit. What to do now?

 

Try replacing plugs.
if no better maybe check out
the #4 injector (maybe a can of
BG44K in the tank before getting
on to that).