ANother thing to check that caused similar symptoms on my MK2 is the Hall sensor wiring. Eventually 2 wires broke completely so it wouldn't even start.
Easy fix to repair the wires and now runs fine again. Some photos here.
http://vw-golf-mk2-gti.blogspot.co.uk/2014/09/vw-golf-hall-sensor-wire-broken.htmlIt is on the side of the distributor so if that has been disturbed to fit new cap it may cause the wires to come loose.
