1987 5000S hard starting, high idle
 

I've been battling what seems to be a fuel problem. It used to cut out
while driving, but no longer. Now it's simply very hard starting -- I
have to crank it for nearly a minute before the engine catches -- and
it idles high at 1200 rpm. Once I'm on the road it runs fine, and it
starts fine after short stops.

This '87 5000S Quattro has lots of miles, and over the past two years
I've checked the mass flow potentiometer (reads a little high) and
replaced:
- fuel filter
- fued pump
- oxygen sensor
- catalytic converter
- spark plugs, wires, cap+rotor.
I also replaced the differential pressure regulator 5 years ago, but
the current symptoms don't match those earlier ones.

Any suggestions?
Thanks, Jaz