I'll offer my somewhat experienced help on this:

First, simply unplug the O2 sensor, and go for a drive. If that makes a
difference, check the wiring and the sensor itself.

Then, I'd start to blame the AFM. There is actually a temp sensor in the
AFM. You can check it out by measuring between pins 1 and 4 in the AFM.
Depending on temp, it should be 1000 to 2000 ohms. There is a graph
in Bentley. Could also be some strange thing with a worn AFM wiper and
temperature.

For all sorts of AFM pics, see my page at:
http://www.kenneke.com/vwfi

After that, I'd make sure all your grounds are good. Bad grounds can cause
all sorts of wierdness.

Finally, check the ECU. I really don't think that is it, but it's happened
before. Swap with a known good one.

It really doesn't sound like any type of fuel issue. I supposed it could
be a bad injector that starts working when it gets warmed up.

Jon

> Hi guys,
> Happy New Year to all.  And when I say guys, I mean the gals too.
> For the new year, I have a puzzle to fix.  I am baffled.  A friend of
mine asked me to help him fix his trouble.  I took it on, but I wish, I
did not.
> It starts fine, idles well for awhile, then vacillates, hesitates, it
dies while driving or while standing at the light, or whatever, until it
warms up.  Once it warm, it is fine.
> Now, I changed the temp II sensor, the Oxygen sensor, looked at the
vacuum lines (although did not check vacuum with gauge), changed the idle
unit that's on the top of the engine.
>
> I have also hooked up, using it the first time, a Digitool.  I get all kind of readings that don't tell me anything, of course.  I don't even know if they are voltages or what.  For example, while I'm getting good reading from the O2 sensor at the sensor, it gives me from 1.2 - 2.2 reading.  About 1.00 more than I would expect.  Or the reading for the O2 sensor ground, which is just over 1.00, up to 1.35.  I guess, it should be 0.00.  But who knows.
> Anyway, is there one online who would know where to look for the fixing?
> I would appreciate it.  Reading the archives did not help.
> Zoltan
>