Okay, firstly, you're about 20 years too late to be taking it to the VW dealership. Dealer techs are typically younger and probably have no experiance with the old style VW fuel injection, plus they charge a lot more than you should be paying. If you're lucky, you live in a town that has an independant VW specialist-- they usually have old timers that have the know-how to really diagnose this thing.
Also, really the only way that owning an air-cooled VW is a reasonable proposition in this day and age is if you do the majority of your own work. There's two books you should probably have, the first being the Bentley Publishing Co.'s service manual for your model, which will contain the information you absolutely need to work on this beast. The other one would be the Muir "VW Repair for the Compleat Idiot" book, which is less useful technically, but is an excellent read nonetheless. Also, the board over at http://www.thesamba.com will have people with more expertise on your van.
It's been a long time since I worked on a old pancake motor, but it sounds to me like you may be having a distributor advance issue. There's some sort of wierd hall-effect thingee that I recall being problematic which may be part of your problem. Hooking it up to a timing light and watching how and if the spark advances would be a good start. Also, I don't know what this thing has for a fuel pressure regulator, but that would be suspect due to the drop in fuel pressure, although the drop in fuel pressure could be due to fuel injectors opening as a result of normal operation. Sorry I can't be of more help!
EDIT: Also, was a valve adjustment part of the complete tune up? Air-cooled VW's are very sensitive to misadjusted valves and they need to be checked every other oil change at least.