Why does my 04 Sierra Denali stop accelerating while im on the freeway and on the console it says "reducing power to engine"?

When "Reducing power to engine pops up on the console, the accelerator doesn't work
so i pull over and turn off the trucks engine and wait a minute and the truck goes back
to normal. At first the Check Engine light would go on then turn off but now its stayed
on. I had the truck hooked up to the computer and they cant find the problem. I've
heard that i have to change the trucks computer?? By the way my truck is the
Quadrasteer Sierra Denali 2004..