check for air leaks @ all the inlet fittings ,listen for vaccum sounds from a hose incorrectly fitted,get someone with a co2 tester to check carbon dioxide levels in cooling system,this will tell you for sure if the new head gasket is sealing correctly,if you unplug 1 injector @ a time you can usualy iscolate which cylinder it is ,if it is only 1 cylinder!did you get the head milled when you replaced the gasket!