has your mk1 got a inline feul filter on the incoming feul line it should be above or beside the left suspension strut as you look at the enginge if not you could fit one to the incoming feul line, you can get clear ones from somewhere like halfords there not very expensive and once fitted you should be able to tell if the crap from your tank is being draged up to your engine as your filter will have crap in it
Harr1s..........