The car starts but misses?

1994 Pontiac Grand Am Mileage: 58,000. My mechanic is a professional auto tech and he has had my car for two weeks now. He has checked all sensors, the computer, the ignition coils, the catalytic converter, and can't figure out what is wrong. The car starts but misses and only goes to about 10-12 mph with backfiring and a burning smell. I believe it is the ignition coils because two of the spark plugs are not firing; any insight would be appreciated.