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.