I know exactly what you are going through - mine has been a pain in the arse for the last couple of weeks, seems to be one thing after another...........Still not exactly decided what to do, been hankering after another Mercedes (my last car) as it seemed a bit more reliable (having said that I spent over £600 on the Merc before selling it for £400!). Cars always go wrong - they seem to be designed that way................
You just have to decide whether to spend money getting it fixed or spend (probably more) money getting another car.
Thing is with Mk2s is they have a way of sparking off conversations with total strangers. It's happened to me twice just this week - both people going on about what great cars they are. Never had a car that has this effect on people, but it's kinda nice and I would miss it............
Nice Mk2s (like yours) do seem to be going up a bit in value, so that is another reason to hang on to it for the moment. You never know, you might get the money you have invested back one day if you play your cards right........