2004 Chevy Silverado

when I am driving down the road the "engine power reduction" comes on and the engine will go to an idle. I turn the engine off, take the key out, then restart. It will go a few miles and do it again. The ownners manuel says it has to do with the overheating of the engine but it isn't getting hot. I just changed the instrument cluster because of the speedomenter wasn't working. Thought that might take care of that problem also but it didn't. Any suggestions?

Thanks,
RJH