Poor running 85 4000q???

[ERIC GLANZ <UCSEDG@uwplatt.edu>]

I am having a strange problem with my 85 4000 quattro.  The symptoms are as
follows:

The car seems to stumble at low RPMs.  When in neutral and I rev the engine,
the engine seems to stumble up to about 2000 rpm, then rev fine in the higher
rpms.

When in gear, at lower rpms, when you floor it, the car will stumble up to
about 2000-2300 rpm, just like it is running lean or the timing is off, then
run perfectly above that RPM range.

It behaves the same wether the car is warm or cold.

This problem is not gradually appear.  On our first cold day (less than 10 F),
it starting happening, and has continued ever since.

Please help me diagnose this annoying problem!

--Eric