I have been having the same prob with my 03 Tahoe, 116K miles. Recently I got a "reduced engine power" message. After reading manual, I checked coolant, slightly low. a week later I got the message again (hadn't even driven 5 miles that day!) Called my mechanic and if I remember correctly he said he adjusted something that had something to do with the throttle (sorry I'm not mechanically minded) Ran great for 3 weeks then yesterday morning it did it twice in the first 10 miles of the day (I was cruising at 65). (I stop and turn it off for a couple of minutes when it happens) I turned around and was doing 75 going home to trade vehicles, it didn't happen. Last night, took it out, within the first few miles it happened. Went on to our destination (only a few more miles) ran fine on the way home. This morning, got in it and there wasn't even check engine light on! Ran great all morning!
If anyone had any additional ideas I would love to hear them! Right now I am planning on taking it in on Monday and having the connections wrapped in aluminum foil! BTW, even though the manual says this is to protect the engine from overheating, the needle on the temp guage never gets past the 1/2 way point.