Recently my '96 XLT 4.0L ohv with 90k miles has started exhibiting rough
idle after what I used to know in my carbureted days as a "hot soak" (stop
of approx. 20-30 mins after reaching normal operating temp).  Engine starts
fine when cold, fine when hot after short stops (gas stop, 5 mins) and fine
after about an hour or more of cool down.  But after a 20 min. hot soak it
has started to stumble and rumble at idle, and even off idle with the
throttle open slightly, for about 30 seconds (it could even be less time
than that - maybe it just seems that long).  After it has "cleared its
throat" it runs fine.  On one occasion it stumbled enough to cough and die,
but after a restart it was fine. Check engine light hasn't come on, and have
not tried to retrieve any codes from the engine computer (something I have
never attempted on this engine).  My impression is that it is running rich,
and my gut tells me to check the evap canister operation - a feeling based
on the previous similar symptoms on my old Chevy S10 would show when its
evap canister went bad and it would dump the fumes at the wrong time -
albeit that was a carbureted engine.  The IAC is new but did not fix the
problem (not to worry, it was already on its second leg after a cleaning
about two winters back, which cured the cold hard starting problem and
prolonged its useful life. Think I got a little too liberal with the
cleaning spray this time and turned it into a trumpet - least that's what it
sounded like at idle after my little "repair" ;-).  It has done this through
at least 4 or 5 tanks of gas now, and seems to be getting worse.

Didn't find much about this googling the groups.  Any suggestions where to
start looking?  Other than this problem, it runs great.

Thanks,
Steve