I think you have to much air or not enough going into the system. (Lean
engine = cutting)
It could be many thing... damn VW fuel injection, simply the worse FI
system i have ever seen. engine temp should be higher if to much air in...

$1500 of various failing parts (AFM, Pressure Regulator, idle stab, control
box, idle switch, failing ECU and so on...)

For sure it's not the Vanagon syndrome.

Pressure valve sound more like it or a hole somewhere in the FI system
hoses or o-ring.

Double check fuel pressure!!!, pump or pump relay could be failing.

Also check your Fuel injector connector for corosion or rust.
How is the OS?, it it working on safe mode???

Regards, Ben 87GL
http://www3.sympatico.ca/huotx/engine1.htm