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You need to start out with a fuel pressure gauge and seee if the fule pressure is not only at spec but does it hold pressure when shut off. If it doesn't hold pressure, you need to find out where it's going. Start out buy pulling the vacuum hose off the fuel pressure regulator and checking for fuel in it indicating a leaking regulator.
If you find no fuel issue problems, take a comple compression test to determine the basic health of the engine.
2003 Jetta left rear window blue plastic part of the