rough idle

My 1991 lumina 76,000 miles has a miss at idle. Runs great at cruising speed. Just noticed today that when stopped I hit the brakes and the engine rpms dropped way off for just a second or two,then returned to tha miss. Then I tried the headlights. Same drop off. The alt was replaced about 2 yrs ago or sooner. Maybe a bad alt?

Sounds like two different issues. Miss at idle sounds like idle air control dirty and clogged with carbon, or possibly a vacuum leak. If yours is a 3.1 and the IAC has never been cleaned start there. Unhook the plug running to it and remove the 2 screws and remove. Don't squeeze the spring assy, just spray carb cleaner on it, and remove the carbon from the head. Also clean the area around where it sits in and run some into the intake after you get back together. Might want to check your vaccum lines for cracks or loose fittings. Look at the intake hose, make sure it has no holes or cracks as well. Obvious things like plugs and wires are suspect as well. You can get alternator checked for free at most auto parts stores, or check it yourself with a volt meter. Check battery voltage before starting engine and then while engine is running. Should read 12+ when off and 13-14 when running. Don't think this would cause rpm fluctuation, though. Vaccuum lines might though.

Do you have a gas smell around the engine?