check engine light and epc come on starting, press the gas and goes off, no code

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When I start my car (doesn't happen all the time) my check engine light and epc light pop on with car struggling to idle. If I hit the gas slightly they go off and everything is fine. I have taken it to dealership, my husband has hooked it up to code reader and it throws no codes! Not sure what it is or how to fix it, obviously it's not right. Any advise?