SLK 230 Check Engine Light/ Head Gasket Repair
 

I have a 2002 SLK 230, my check engine light is on and when I took it to a mechanic to diagnose the code I was told that the code was Misfig # 3 also the mechanic notice there was steam coming out of the head gasket and stated it needed to be repaired or replaced. I recently had the water pump replaced by the same mechanic. What's interesting is the car runs smooth when it is at least 1/4 of gas in the car and anything less the car shakes and makes a loud noise. Can anyone give any insight to the above and an estimate of estimate of how much it would cost to get it fixed? It would be great if someone knew a great mechanic in Pasadena, CA. Thanks