Here's a rambling thread where some of the basic electrical cleanup and some mechanical work is being done on 84 Euro. In this case, main issue was computer grounds and crank position sensor, one of which was causing intermittant miss. Point is that this will all be subsets, especially the electrical, of stuff you will need to do to your car to establish a solid baseline from which to get it running reliably.
There are several owners here of 84/5/6 16V Euros and folks knowledgeable in 85/6 32V USA which share very similar engine management system that can help you. Each of us may have a different approach though.
http://forums.pelicanparts.com/porsche-928-technical-forum/586148-ground-cleaning-16v-euro.html