Several ideas, course we all describe things differently. if it is lacking fuel it will kinda stumble around after you mash it then all at once pick it up and be gone. anything from bad filters to inlet lines hoses sucking air or coming apart causing blockage. Does smoke accompany this problem, have you tested the anaroid. Did it show up after it was worked on for something, or gradually on its own. The line cming into the lift pump from the filter base is known for coming loose at the pump side and sucking air. Also could the valves be set wrong, worked on one last week the flywheel wasnt in the right position and the valves were set with it pinned which was bad out and it showed lol. have you checked the timing?
If no smoke, it's filters. Mine did this too. I'll try to post the pics of my filter guts if I get a chance.
