I'm guessing it's compression related, do a compression test.

If you think it's fuel related, buy 4 cans of diesel purge, and run straight diesel purge under load at WOT (full rack) on in incline such as a short bridge that separates two boulevards. Make sure the engine is at operating temperature so the diesel purge can work it's magic.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.