HERE'S ONE THAT'S GOT ME STUMPED.BEEN GOING ON FOR A LONG TIME. SOMETIMES NO THROTTLE RESPONSE AT ALL, OTHER TIMES NO INCREASE IN RPM, JUST REAL BAD LOPEING. ASSUMING A FUEL PROB, I HAVE TRANSFERRED EVERY FUEL RELATED PART FROM A SPARE TRUCK THAT WORKED FINE. MIXER, GOV, REGULATOR, FILTER THROTTLE BODY, GOV CARD UNDER DASH, GOV SOLENOID, VACCUUM HOSES ARE ALL OK, ECM,. THERE IS NOTHING ORIGINAL LEFT ON THE TRUCK. ALSO FOUND A BAD GROUND & ONE BAD CONNECTOR. THIS TRUCK IS RUNNING ON CNG. WORST OF IT IS THAT IT CLEARS UP AS SOON AS I GET IT IN THE SHOP SO IT,S HARD TO DIAGNOSE. DOESN'T RUN BAD ALL THE TIME , JUST OCCASIONALLY WHEN IT'S RUNNING IT RUNS GREAT. ANY IDEAS !!!!! THANX