my 1996 mercedes benz s500 with 100k miles on it set up for about 6 months and when i started it up, it started to idle up and down like it wanted to cut off.whenever i crank it up, it stalls and then runs perfect goin down the highway sometime it will die down for a second or two then pick rite back up.. it use to put off this horrible gas smell like rotten eggs, but the smell finally stop coming from the exhaust pipes after about 5months.. it does this in the winter and in the summer. i have never replaced the plugs or wires. i've added fuel to it time after time throughout the last 2 years. i run 93 octane in it.. whats my problem?