[V8] Low idle

[QuickAudi at aol.com]

Ok guys, my idle is really messing with me lately.  143k on the '90 V8q.  At startup before, it was a high idle, around 1200 or so.  Now, it is a low idle, around 600 or so.  I am not sure if it got that way after I had the two engine sensors replaced or at some random point afterwards.  Could the sensors being corroded or loose or put back on the wrong wire make this problem occur?  If not, what else causes a weird idle?  My idle control valve is clean and fine and working, according to VAG-COM.  Oh yeah, no codes, either.  Vacuum leak?  Where would I start checking?  I notice that when I leave it idling that it seems to run rich, or maybe it's just my imagination.  
On the list to replace is the plug wires, distributor cap/rotor, maybe the MAF, might remove the K&N.  Any thoughts?

TIA

Joel
'90 V8q
'86 5kcstq