Have you tried having the car set up again, did you alter the timing when changing the dizzy cap? if its even slightly out they run like a pig, look for the set up guide in the mk2 maintainance section, that would be my first port of call. Failing that... maybe try the blue temp sender? When mine was bad all the above were shot, I changed the coil as well (about £20 from GSF, temp sender is about £12 from VW) mine runs much better now, used to feel really flat before. A long shot might be a new fuel filter? Have you checked air filter/throttle body for congestion? Lots of things you can try really. Process of elimination I guess.
