I'm experiencing the same problem I have an 04 jcw mini light mods, well taken care of etc, 80k, if I drive like a senior citizen no issue, I clear the codes and go about my day, but if I exert any load on the engine in excess of around 3500 or 4k it will misfire specifically multiple misfires, initially was cylinder one has now spread? Wtf is going on I have changed the coil pack it had an msd went back to stock, new wires plugs, compression is 160 across the board, fuel pressure is consistent and within norms. I am at a loss here, I ran seafoam through it and replaced the air filter and cleaned the throttle body etc. Help a brother out with this mysterious dilemma if you ever resolved yours. I have not checked crankshaft position sensor, or the injectors them self could probably be upgraded my plugs looked like they had been running a little lean, I haven't hooked up a wideband to it in awhile though.