Keeps saying reduced enigne power

I'm having the exact same problem with my 2003 Tahoe. It doesn't seem to have a pattern - any temp outside, cold/warm engine, in-town/highway speeds. I have to pull over, turn off engine, sit for as little as a minute or two, and then Tahoe starts right back up and runs normally, except the check-engine light is on. After a while (no pattern), the light goes off and it runs fine for either a few miles, or a couple of hundred miles...again, no pattern. Hopefully someone can shed some light on this!!

Throttle body harness