Have you pulled the valve covers and looked at the valve train yet ?? Could be a couple of bent pushrods, cam lobes going away ( remember that hard starting ?), Broken valve springs etc.
Helped a guy recently with very similar issues (via phone), wound up being the camshaft going away.

Another thought might be to try a vacuum guage although it will likely just tell you what you already know, you have several weak cylinders. The question is why?
Keep us posted