V8Q--Warm start idle problem

[Mike Kerrigan <mk85911@ibm.net>]

90 V8Q with 93k. If outside temp at least 50 degrees and engine warm,
the engine will not hold idle rpm's and stalls. If gas pedal slightly
depressed, after about 30 seconds or so "something: kicks in and idle
becomes solid at about 700-800rpm. Problem never occurs when engine
cold  and never occurs when outside temp below about 50f.
   Problem started after dealer replaced timing and serpentine belts.
Dealer has checked for faults seversl times, but none indicated. Dealer
says it can't be improperly installed timing belt because engine would
not run at all. Is this correct?Is anything disconnected during belt
replacement which may not have boon reconnected? (I haven't noticed
anything). 
   In the interim, I have replaced spark plugs and O2 sensor, but no
improvement. Dealer suggests that idle air valve or idle stabilizer
could be at fault. Also, I noticed some postings a couple of months ago
concerning default modes ( associated with the O2 sensor )being
determined by coolant temperature levels. I would greatly appreciate any
thoughts--thanks.

90 V8Q
85 911
MZ