I have an '89 RS 305 TBI auto. I guess I'll just cut to the chase. I usually put Chevron 88 or 91 octane gasoline in the car. I did that for three months straight recently and had no funny business whatsoever. One day I was in a hurry and had to put 89 octane Fina fuel in it. Everthing was fine until the next morning when I started it. It ran great at fast idle for about 2 minutes and then all of a sudden revved up to about 2500 rpm by itself briefly and then shut off. !!! what the hell!! Is it posessed or something?

It was doing some funky idling a while back (erratic bursts from 700 to 1000rpm) and one day it died on me and wouldn't start again. When I cranked it I could smell a sulphur-type smell. Replacing the ignition module fixed that.

The new problem is similar but only happens when I use a different brand of fuel. Has anyone heard of this before??

 

Your problem should not related to the grade of fuel you use. FWIW, you only need to run 87 or 89 with swirl port heads. In case there is some bad fuel in your area just run the same grade (from the same place or chain) for awhile and see if you can develop a trend. A rich sufler/rotten eggs smell and a bogging motor is a sign of a dying catylitic converter.