modifications are 3" pypes exhaust,no cat,3"cold air intake with 3.5" ls1 maf and translator.afpr,50 lb injectors,power plate,and cx racing t66 turbo.i have no idea about the internals of the motor or tranny.the popping and backfiring is occurring while driving the car and the tranny is in fourth gear.if I let off the throttle and let it go back to idle,no foot on the pedal and the car is just rolling say at 55 mph and I go to push the throttle back down,foot on gas peddle,as soon as I start to push on peddle,it will pop and or backfire,if I let off its fine or if I push the peddle down further,it will clear up and is fine.only does this in fourth gear,does not pop or backfire in any other gear and does not do it under boost.i will disconnect actuator rod today and check wastegate.thank-you everyone for helping out.