with my 3.0 ranger, I had it pull a valve up into the head to where it was recessed in the head itself and that caused a misfire. Replaced the heads with a set from another ford 3.0 and fixed the problem 100% also another but more expensive possible fix. the reason why mine did it is because I was running at too high of RPM with too little of gas being pumped into the motor due to my supercircuits tuner.