bubbles in fuel

[priya]

While traveling on a road trip my 85 300TD wagon started slowing down on hills. It seems the problem may be air bubbles in the fuel line because when I rev the engine the primary filter gets extremely foamy. I tried bleeding the air the normal way with the primer and loosening the top bolt on the fuel line but after the air seems cleared out the whole thing begins again. I checked connections at the tank and at the filters but all seem tight and with no leaks. The weird thing is that it is intermittent—like day to weeks before it occurs again. A possible clue: Before this all happened, a mechanic that blew air back to the tank as a kind of maintenance.

Also, the fuel flows regularly out of the tank (I disconnected the lower hose and checked)

any thoughts?