2008 gmc 2500 reduce engine power

[Wes1978]

First time posting here, I’ve got a 2008 gmc 2500 6.0l, very very long story short I did a variable valve timing delete , mild summit cam, headers and injectors, I’ve had this thing to the tuner a dozen times and when the truck is running it runs like a bat out of hell but that’s only when it’s not going into reduce engine power randomly, I’ve replaced the wiring that goes to the throttle position sensor , the Throttle position sensor twice, gas pedal. My tuner  can’t seem to help and he’s a reputable intelligent guy, if I could figure out how to control the 6l90 transmission I’d do a carb swap on this thing………… the only thing I’ve been able to figure out is when I’m driving down the road and it goes into reduce power I’ll pull over and relieve the fuel pressure by pushing the schrader valve on the fuel rail and it comes out of reduce power every time, I don’t know if that has anything to do with my problem ? Fuel pressure 60psi.

[CamGTP]

What code does it throw?

[Wes1978]

P2135