HELP! '95 SL500 backfiring, stalling

[raftel]

Decided to take my mother's '95 SL500 with 53K miles on a short run. It hadn't been started in six weeks. Clear weather, 60 degrees F. For the first ten minutes, running through light traffic, it was fine. Then traffic started getting heavier, and it started to stutter. It got to the point of being labored leaving a light. For the last part of the trip, it was backfiring, felt like not all cylinders were firing all the time, and would not pick up from a stop. It also smelled awful - a rich/sulfur smell.

No CEL, oil pressure/water temp were fine. Is this just a question of old gas and not using the car enough, or do I have other issues to look into? I'd really like to get this car ready for the season.

Thanks!