Has the check engine light come on? Have you checked the catylatic converter for plugging? Has the fuel pressure been tested? And finally, (I can't remember) is the 86 fuel injected or does it have a carb? I remember having trouble with the carbs coming loose and causing this type of problem. Also, check for vacuum leaks. If all checks out ok, I would run a compression test. That is a lot of miles for a 4cyl in one.