Pull the plugs and look at them. Reading plugs tells a great story. My guess is if you haven't tuned it up for 5 yrs. it's time. The older stuff didn't run nearly as long as new cars do on a tune up.
Does it sound like fuel or electrical? Time for a full tune-up? Timing need adjusted? Carb need rebuilt?
Out of all the things you listed in the above quote the only one I can most likely rule out is the timing needing adjusted. Why you ask? Once timing is set it changes very little. Not generally enough to cause this problem. Unless the dist. hold down is loose of course. It never hurts to check the timing though. Any of the other things could easily be the problem but I'd start at reading the plugs first. You might pull them out and find them burned to a crisp cause their so old.
