2002 Cougar, rough idle, stuttering when defrosting

[MightyPirate]

Okay, this is an annoying one:

2002 Mercury Cougar, 105,000 miles, bought it used about 4 months ago.

Symptoms:
Runs great except, when idling it will skip about every 10 seconds or so (comes and goes sometimes, only happens in gear, either drive or reverse). Happens either warm or cold, also it happens A LOT more when I turn on the front defroster. Also, if the defroster is on I can hear/feel it stutter during cruise (any speed). The stuttering increases with more fan, about every 3 seconds if on highest fan setting. Does not matter if cold or warm air.

Also, the other day I looked at the spark plugs wires in the dark and there is some moderate arcing in the wire, I'll probably have them replaced.

Other symptoms are problems downshifting though this may be a tranny problem (no smokey or burnt fluid yet and it shifts fine, only downshifting under heavy load like going up a hill). Shifts nicely if I push the gas pedal a bit past the point where it would be just about to shift.

Repairs done (in no particular order):
Changed fuel filter
Cleaned/Checked MAF sensor
Checked fuel pump (it has the new cougar pump, not the old bad one)
Had dealer plug it into computer, he said it was OK (and then charged me $150 )
Changed oil and filter, put in synthetic 5W-30 oil
Checked air filter (only minor dust, not worth changing IMO)
Checked hoses for cracks, none found
Checked front 3 spark plugs, no problems and they looked recently replaced so I left the hard to reach back ones alone.
Checked the battery
Put in dry gas and fuel injector cleaner (not at same time of course)
Checked belt tensions, all good
Cleaned battery terminals
Reset computer
Listened for engine noises, none that I can hear
Checked fluid levels
Checked overall engine for damage and it looks to be in great shape.

This started happening about 2 months ago and has not really improved or gotten worse. I would like to get this fixed as I like my cars running smoothly.

P.S. Anyone know where the PCV valve is on the cougar, I can't seem to find mine.