[TominMO]

So there are basically six areas to consider: fuel, air, spark, valve timing, valve lash (gap), ignition timing.
EDIT: Adding sufficient compression to list.

-- Air is rarely a problem, unless the filter is insanely dirty or someone stuck a potato in your exhaust.
-- Clearly you're getting fuel and air, since it runs for a while.
-- If valve timing or ignition timing were off, it would run very poorly or not at all.
-- So to me, the most likely issue is valve gap. Check it and report back. When was it last done?

But another issue could be a partially-clogged fuel injector; i.e. good enough to get it running, but not run well. Wouldn't hurt to dump in a small amount of fuel system cleaner. I'd do it for a tank of gas. If the bottle says it treats 20 gallons for example, I'd use 1/10 to 1/8 of the bottle for two gallons. In my experience with cars, fuel system cleaner takes about 100 miles to do the job. At that point it will run smoother and stronger.

Always go with the cheap and simple fixes first.