I once had a 89 BMW 535i with an erratic idle. Sometimes lumpy, sometimes fast, and sometimes it would stall.
I chased all over the engine bay looking for the fault, replacing more or less every fuel/air related ancillery to no avail.
Turned out it was the ECU. Got another from a scrap yard and it still wasn't right. In the end i bought a used one from Ebay and it sorted it out.
15-20 years in hostile conditions can play havoc with old electrics.
Don't like the sound of those fuel lines though, can't be doing the system any good. Have you had a quote for a set to be made up?
Daft question, but i suppose GSF/Euro parts etc don't have them?
If you're certain your systems are in good order, then the next logical step would be the ECU.
If you can't find another one, carefully, and i do mean carefully, take the old ones case apart and you should find an old fashioned curcuit board inside. Get a magnifying glass or some other magnifying device and check carefully over the board checking for dry joints, corrosion, signs of burning or broken tracks. Take your time about this as it can be tricky to spot. Anything that looks suspect, waft a soldering iron over it and maybe chuck a bit more solder into the joint. If you can, try and have some flux on hand to help the solder flow properly as old solder can be a bit stubborn.
This may all sound a bit extreme, but if you can't get another ECU then repair is the next best thing. I've managed to repair a couple of ECU's in this manner plus a few instrument binnacles too, so its worth a try.
Alternativly, there are companys who will test and/or repair your ECU for you. Last time i looked it was around £100 odd.