I'm by no means an expert but your experience sounds similar to mine. As it turned out I had a failed ECU. Always started fine, would sometimes idle okay & run for a couple of minutes but invariably would run like crap shortly thereafter, with no power & terrible misfiring. I replaced the AMM but no improvement. The difference is this, though: my car is LH2.4 whereas yours is, I believe, LH2.2 and I don't think the ECUs were quite so problematic on those, not with that failure mode anyhow. Have you verified that the fuel pump relay is operating? If I recall correctly one failure on the 2.2 ECUs is a loss of signal to the fuel pump relay. If that's it see the FAQs for a couple of workarounds. If you've got LH2.4 and it's got the 0 280 000 563 ECU, look for a 0 280 000 953, I believe that's it but I'll double check it for you, they're apparently less prone to dying than the 563s. Good luck!
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina