2002 Mercedes Benz Sprinter misfire lack of power

It is a petrol lpg, it will not rev over 4,000 when driving along, it can be okay one minute and then next misfires and runs as if running on 3 cylinders, lack of power.

Have changed spark plugs, ht leads, coil packs, throttle body, air flow sensor, changed air box sensor. It cannot possibly be the injectors because when it misfires I have switched it over to petrol and it seems worse, injectors would only cause problems on the lpg side.