When driving my 2005 Chevy Silverado, 2500 HD, 6L the "reduced engine power" comes up on the dash. I had the local mechanic connect his scan tool to the truck, he said to replace the accelerator position sensor. So after replacing 2 of them with no correction to the problem. I took it to the Chevy dealer, they recommended replacing the wiring harness to the sensor. Two weeks after replacing the wiring harness the reduced engine code came up again.

I have a CarMD scan tool. Error Code P1125 comes up when plugged in.
The definition of the code stated : Accelerator Position Sensor 1 and 2 Circuit Malfunction

 

If infact a dealership diagnosed and replaced the harness, then in my opinion, if falls under warranty. next step might well be the Engine Control Module.

 

i hate to say your computer may be going out
does teh security light come on when this is happening
we had to replace 2 computers in one work truck cause it did the same thing yours did