2005 Chevrolet Cobalt

Shakes or Wobbles problem
2005 Chevy Cobalt 4 cyl Two Wheel Drive Automatic 138000 miles

A few days ago my 2005 Cobalt started shaking pretty bad when I pulled up to a stop light. When I tried to accelerate I couldn't get much power. I put in a fuel treatment, changed the spark plugs and the air filter and for a day or so the car ran fine. Today it started again but this time it didn't kick back into gear as it did before.
I also smelled something burning under the hood but couldn't pinpoint where. Had a fuel system test done on it but no leak but I did smell fuel as well before I had that done.
It's very frustrating.
Engine lights been on for a while now. Had 2 diagnostics done on it but nothing could be found.
One mechanic says I need a muffler. I really don't know where to go from here. I really don't want to get another diagnostic if they can't pinpoint the problem.
May 28, 2010.

Check out the related content below while we wait for the question to be answered by a professional mechanic.