My boyfriend has a Pontiac Firebird and it has been missing a little every once in a while when he is driving it. It would miss when stopped at a red light or going on the interstate. Yesterday the car was missing pretty badly and had a loud sound coming from under the hood. He last year changed the coils and also last year changed the spark plugs. Do you have any ideas what it might be?

Check a spark plug and see if its fouled.

Is the service engine light on?

Make sure the plug wires are on good. Maybe they need to be replaced.

Also try some injector cleaner, he could have fouled fuel injectors. Check the filters as well. Should be in either the fuel or primary electrical systems.