One thing i would try and this cured a PB engine Gti with similar issues. Go straight to the ECU and find the earth. cut from main loom and then run from ECU to a solid well known Ground point and try it. Dont loom it up this stage in case of no fix but just run a length to ground. High resistance over time can affect the ECU. They are very tempremental. And age does get the better of them. Sounds like wiring as you say the lom is ratty. I would try the earth thing man. The Golf i did has been sound ever since, this too is digifant, Trans is digifant (early) or SIMOS.
GIve us an update.
CHeers