My car was running perfect untill a week ago. When the car holds full boost (12psi) it will make a VERY loud bang sound comming from the exhuast and the whole car would hesitate. The nexy day I changed my spark plugs and the problem went away 100%. Well a couple days later it started doing it again. Now it rarely makes the loud bang sound but the whole car hesitates. My LINK ECU is saying there is 0 knock. Its a 1990 FM2. Any suggestions?