I had the exact symptoms you describe. They continued for several weeks until the
engine finally died on me and wouldn't restart. After consulting folks on this forum, my buddy and I spent the whole day methodically tracing the
fuel system every inch from injectors back to the tank pickup. It turned out the tank pickup was clogging with trash. There were also a couple of loose hose connections between the tank and the
engine. When the tank pickup clogged, air would be sucked into the
fuel system through the loose hose connections.
I think the most likely place to start looking is the tank pickup.