91 200q Stumbling When Hot

[Sargent Schutt <sargent@novagate.com>]

Houston, we've got a problem. And it has nothing to do with the first AARP
member in space...

I've been trying to figure this out for a while. Unfortunately, being short on
time and $$, I haven't been able to test much myself, or take is to the
$dealer$. 

Situation is this: 

Air temp ~60-70 F, sea level. Drive 5 min, let oil temp rise over 60, then
make a couple of extended full throttle applications. On the second 0-70-90 or
so run, the car will stumble and stutter on full throttle application, like
pinging, I think. I have occasionally heard a backfire on a 2-3 shift. 

Then, after getting on the freeway a mile or so (effectively cooling it off a
bit), reapply the throttle (say 4th or 5th gear), and full boost comes on
without a problem. No bumps or bobbles. Full 1.7-1.8 showing on the gauge
(modified computer mit 2.5 bar PT, 2.2 cutoff).  Keep it up, though, and the
stumbling returns. It only has trouble on repeated/extended full throttle
runs. Let it cool down, and things are fine again. 

What gives? I have, in the past two years, replaced plugs, wires, cap, rotor,
air filter, O2 sensor, complete exhaust from the cats back (incl new cats). 
The oil is good and clean (as always), though I did overfill it at the last
change and subsequently had some black smoke on startup. That's about gone
now, though. And the problem existed prior to the 1/2 qt overfill, just the
same. 

Stats/mods: 183k miles, 40k on mods (bone stock when I bought it at 143k) TAP
chip, stock WG spring (screwed down 2 cm tighter to compensate for any spring
aging, didn't seem to have an effect), Stebro cats and dual 2.5" exhaust (60%
improvement over stock on Stebro flow bench, OE manifold/downpipe), K&N filter
with open air box (front open, extra heat sheild keeping hot air from
turbo/manifold away).  These mods are not that radical, but, for
consideration's sake, that's what's been done. 

So, perhaps:

1. Knock sensors? (Would repeated accel runs (heat) affect these, especially
if they've not been replaced in a while (ever?).
2. MAF sensor? (what to do with repeat/heat, I don't know, should not matter)
3. Plugs? (they have ~40k on them, the proper plugs, as recently discussed,
what's heat got to do with it?)
4. Turbo heat soak/overspinning? (spins freely, spools quickly, I did think
this might be the problem -eg 60% improvement in ex flow from the cats back,
perhaps overspinning and need K26? Seems that should be okay, from the
response I received)
5. Fuel Filter? (Has ~70k on it, I'd guess - needs changing anyway,)
6. O2 sensor (Idles fine).
7. Fuel pump?
8. Injectors?

Any BTDT? Again, the hotter/harder the car is driven, the worse the stumbling
sensation gets (as a function of time).  It is also a function of boost, as it
will only stumble when the throttle is applied past 75%.  If allowed to cool
off (while driving), the problem goes away, but a minute or so of hard
acceleration brings back the stumbling. 

I really appreciate any insight here. I'm not too sure where to begin. I will
be chasing this down next week, in preparation for a  week on the road. 

Many thanks in advance,

Sarge

91 200q 183k, TAP, Bilstein/Eibach, Stebro, Euro's, No Bozo Audio, etc etc.