Pontiac Repair: service engine soon, costly repair, computer scan

Question
02 firebird v6 S.E.S. light is on stays on steady at idle, runs rough like it's missing and low power. the light when moving sometimes flashes or goes out but car stil runs rough. exhaust has a very sulphery smell also.  

Answer
Hi
To properly assist in diagnosis I'll need the Code number for the Service engine soon light. You can go to most large chain parts stores and car care centers and get free Computer scan.
I can tell you this, it PROBABALLY is a misfire code so you should consider a Major tune up, including plugs, plug wires, fuel filter, air filter, throttle body clean and injector flush. Also have the Coil packs checked for not having terminals rusted.
When the light flashes, it is a Catalyst damaging condition, and should be checked ASAP, so you do not damage the Catalytic Converter. If it is damaged, it is a costly repair.
Hope this helps
Just Advise from experience.
Good Luck
Paul