sometimes when u turn ignition switch on nothing happens,

What is your question?

othertimes engine turns over but does not start,when it finally starts code on dash says engine power reduced and check engine light stays on. Chevy dealer is 30 miles away and by the time I get it there it clears up and check engine light goes off. dealer kept it for 2 wks. and cannot find the problem. I contacted GM tech support and they have no idea. vehicle has only 6700 miles on it