1993 Jimmy T15, 4.3L L35 engine (VIN W).  Starts/idles fine cold.  Hard to
start hot.  When hot, if you slow down and stop to park from any reasonable
speed, shift into park, and release the regular break, it cycles in idle
speed a few times (down below 400, then up to 700, then back) before
stablizing.  Similar effect but much less pronounced when stopped at a stop
light with the engine in drive.  Any idea on what the cause could be?  

Checked PCV valve for vacuum, is fine, swapped with another new one as
well, no change.

Linear EGR was replaced a few months ago by a local garage, after it
suddenly had difficulty running (and starting), and MIL light came out
almost immediately.  This time, no MIL, and no stored codes either.

Engine seems to be running rich (from standing outside when running) at hot
idle.

Also, when started cold with the hood open, I noted a bit of a sound from
the alternator (a scratchy sound), but it is not very pronounced.