Check your sear. If it is sloppy and have a lot of play, under a heavy upgraded spring it can let go the piston prematurely. Unless the piston is soft flexible plastic, it can hold provided the sear can hold it.
Look at these videos which shows the sear slop.
http://www.youtube.com/watch?feature...&v=TmBhCv8xPxk
M28 cnc aluminum sear set for Echo1 - YouTube