1999 Pontiac Firebird Car shakes violently, check engine li

Hi,
I have a 1999 firebird, with 96,000 miles. It is my baby, but it has been costing me quite a bit of money lately and I was hoping I could get a couple of suggestions here.
When I am at a stop light, my car will begin to shake very violently. My check engine light will come on and blink at me. It will shake so violently, I somtimes think my car is going to die at any moment. This will happen for a couple of days, and then it will go a week or so with out any troubles at all.
When I have had the code read at the autoparts place, it says something like multiple missfires. My mechanic has looked at it and told me it was something with my ground wire, but that has not fixed the problem, as my car still shakes all the time.
I have had all spark plugs and wires leading to them replaced. Just a couple of weeks ago my car was overheating and I had about 600 dollars worth of parts and labor done to fix that, but still have the problems with the shaking.
I dont know what else to check or replace, so was hoping someone could give me some suggestions or a place to start.
Thank you