This is making me nuts.

When 'Roland' (Wolfsburg Westy, 1.9L Automatic, 57K miles) and I began our
relationship almost two years ago he was showing signs of neglect -- not bad
but neglected.  Power output was pretty sad and he idled like a Harley.
First shot was new cap, rotor and plugs -- solved a bunch of problems.
Second round was a new Idle Stabilizer, Digitool (nothing to do with running
but I did upgrade from a Triumph-Adler ECU to a Bosch in the process), Temp
II sensor and throttle switch adjustment -- again a big improvement.  This
fall I installed a new exhaust system (coated), muffler and O2 sensor --
again big improvements.  When the van is cool the throttle response from a
stop is outstanding but once the van is warmed to normal operating
temperature it stumbles and backfires unless the full throttle position is
engaged.  Even at full throttle it stumbles briefly before something clicks
(usually takes about half the width of an intersection) and it takes off.
This only happens from a full stop.  Normal slowdown-speed up throttle
response is very good.  Highway manners are excellent -- good power and good
mileage.

Went through the Texas emissions test with flying colors.  Very low numbers
across the board.

Last weekend I adjusted the idle speed and timing to Bentley specs.

Last night I cleaned the AFM wiper strip and shifted the card ever so
slightly.  The quality of the idle improved but once warmed up the stumble
returned.

What should I be looking at now?

In case it matters, I have installed Bosch Platinium +4's, K&N Filtercharger,
Turbomax 14771 Muffler, and polished the throttle body throat.  The rotor and
cap have about 4K miles on them plugs have been in about 10K miles.  I've
checked and tightened all the intake related hoses and fittings.

Chuck