What can I expect in cost to fix or is it simple like giving it better gas during this high heat?

I have a 2002 turbo sport vw beetle with approx 53,000 mi. Today, 102 degrees on the way home I had trouble getting up a hill in second gear with the a/c running (usually no problem) and I commented that I didn't like the way the car was running. After parking for an hour, the car was more sputtery and sluggish than it had been on the earlier trip, and the check engine began flashing. Turning off the a/c and letting the car idle caused the light to turn off. I smelled rotten eggs and a burning smell on my 1 mile journey home, but cannot be sure they were necessarily coming from my car. I am wondering what I might expect in terms of the problem/possible cost. I intend to tow the car to the mechanic. I may have put the mid-grade gas in it this last time, and I usually do the hightest.