stannt

2003 Avalanche 5.3 with prince lpg. Having misfire on cylinder 8 almost all of the time. Can run fine for several miles and suddenly start running very bad. Misfire vary from barely noticeable to almost stalling engine. Sometimes running fine on lpg and bad on gas. Sometimes the other way around or bad on both. Only code i can find is cylinder 8 misfire. Compression ok on all cylinders. Engine light does not light up before engine runs very bad. I have tried switching to new o2 sensors, new injectors, new spark plugs, new coil, new trottle body, new spark wires?) and new inlet gasket air) Anyone have any ideas for where or what to check next?