misfire/spluttering

BIT OF HELP NEEDED,CAR HAS DEVELOPED A MISFIRE WHEN BOOST IS TURNED UP,STARTS AND RUNS FINE NO PROBLEM ON STANDARD BOOST AND WILL ACCELERATE NO PROBLEM AS LONG AS BOOST IS NOT INCREASED,BOOST CONTROLLER IS A SPEC S BLITZ AND APPEARS TO BE WORKING OK,WITH CONTROLLER ON 0 SET AS SAID THE CAR WILL...