I wouldn't concern myself too much about the quality of the fuel - your problem seems mostly to be related to heat issues under the hood.
An ignition module that is on its way out will appear to be bad when it heats up - causing your stalling when your going down the road for no apparent reason, not starting again until the engine bay cools off. On the days that it just doesn't work at all you go nowhere.
The problem is that a replacement ignition module costs $100-$150. Even a marginal module will appear good when tested most times - but it is worthwhile to have the mechanic read the codes from the computer. Some people say that you should just replace the whole distributor - I have heard of prices for a complete distributor in the $200-$250 range. Another option would be to go the salvage yard and pick up a used distributor - you can get the whole thing for $75. Have a look at
www.car-part.com and do a search in your local area.
Good luck