2007 GMC Yukon Engine problems

My traction control light comes on, messages service stabilitrak and traction control system are displayed and my check engine light comes on while my Yukon is Idling. When I start driving, the vehicles shifts roughly from 1st to 2nd gear. After that the traction control light goes off and there are no service messages displayed, but the check engine light stays on. The fault will only duplicate itself when the vehicle is idleing. My mechanic is currently working on the problem. The are saying that the service engine light is on because of a misfire, and the stabilitrak problem is a result of that too. It doesn't sound right to me but because I am not to familiar with engines, I have to give them the benefit of the doubt. Hopefully one of you will be able to help me out before I drop a ton of money