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When I start my car, if it runs for more than a minutes (I like to let it "warm up"- habit from cold winters) it shuts off; I took it in for service, a couple of weeks ago and they ran a computer diagnostic test ($93) but didn't find anything. They kept it overnight and said it was fine. Today, I stopped at a stop light and it shut off and wouldn't restart. After two tries, I was finally able to get it started, I quickly put it in drive and shot off, luckily I was only a few blocks from home. What could be wrong and why didn't the repair shop find anything?