Stalling

I have a 2004 Trailblazer that stalls and loses power. The dealership upgraded software several months ago but problem returns from time to time. The engine will idle at 900 rpm instead of the normal 600. I get power loss at steady highway speed. It has stalled while turning the wheel and also on highway last night. Very scary feeling. The problem seems to disappear when I go to the dealership. It has only got 7000 miles on it. Any suggestions as to the possible problem.