[s-cars] Need time for S4, help me fix my 200!

[Rich Assarabowski]

Not strictly S-car related, but the sooner I fix this one the more time
I'll have for the S4!

My 1990 200q suddently started running poorly.  Engine runs fine when
cold for about the first minute, then idle becomes very rough, stumbles
at the slightest application of the throttle and has very little power,
car is barely driveable.

Since my fuel pump had been very noisy and was due for replacement
anyway, I put in a new pump with no improvement.

I have done the following so far:

-- System pressure, control pressure and residual pressure all within
spec (engine hot or cold)
-- No fault codes (reads 4444)
-- All solenoids click when running output checks (O2 sensor freq valve,
wastegate freq valve, cold start valve, carbon canister freq valve,
carbon canister on/off valve)
-- Plug wire and rotor resistances all OK, swapped each plug wire
individually with a known good wire, no difference, plug wires are
relatively new
-- Replaced plugs with another set, no difference
-- Engine compression good (140-145 psi), hot or cold
-- Removed injectors and checked for pattern (engine cold), looked good,
no leakage
-- Replaced injector O-ring and tip O-ring in each injector
-- Checked cold start valve, pattern good, no leakage
-- Idle stabilization valve clean, opens and closes fine with 12V
applied
-- Michelin Man hose recently replaced
-- Throttle switch recently replaced, was adjusted properly and worked
fine
-- Checked all vacuum hoses, all appear OK (but you never know!), found
deteriorated WG to intake manifold hose, replaced it but no improvement.
All other hoses appear in good shape, some have been replaced in the
last couple of years.
-- Disconnected O2 sensor (to run open loop?), no improvement

I'm somewhat stumped right now on what to do next, any thoughts?  I'm
continuing to look for a vacuum leak, especially in the deceleration
valve area, which is very hard to get to.  Could some of the sensors be
bad even though no fault codes are displayed?  Any particular areas
prone to vacuum leaks?  Would the fuel distributor be bad even if system
pressures are OK?

-- Rich A.
  '90 200q
  '93 S4
  '83 Coupe GT