Not sure what your problem is, but I'm fairly certain the TDI's use the injector pump to get fuel from the tank - in other words there is no in-tank lift pump. Is it possible that either the pick up pipe in the tank is partially clogged/collapsed or the fuel tank vent pipe blocked? These symptoms 'may' respond to a thump of the fuel tank. Other than that check the fuel line junctions ( and lines themselves for that matter) near the tank since even a tiny air leak would cause fuel starvation.
Best of luck
John
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