Wings19

Hello all, Some I'm here well, because I have the dreaded reduced engine power mode. So here it goes. 2003 Silverado ls 1500hd crew cab 4x4 6.0l Vortec. I have been tracking down to no avail and still nothing. I have had the codes p1518, p0220, p2135. I have started out by change my throttle body and sensor, no luck. Changed out the TPS harness and solider and shrink wrap and no luck, changed out pedal sensor and still p2135. I ordered and installed a new PCM well no luck. I did a Voltage drop test and found a broken ground and repaired that. I have traced wires and cleaned and fixed all grounds wires. With all the new stuff on the codes will all clear and it stayed off till i put the truck in revearse. Then the good ol' ding ding REDUECED ENGINE POWER. Ever now and then the only code I will get is p0220 ( Throttle/ pedal position sensor/ switch B circuit). I no longer get the p1518. Once in awhile I will get the p2135 as well as the p0220 codes. I'm at a loss right now. Could I have a bad Throttle Control Module? Is there a way to Test this TCM? Any and all input and information is more then welcome and im opened minded. Thanks in advanced