Air coming out of plenum

Hello. Working on a 2000 Ford Taurus V6 3.0 SOHC (I believe). Long story short - power steering pump went out. Vehicle overheated briefly, started running rough was first clue. Pulled over, let engine cool, radiator fluid low, added water, went home. At cruising speed, car runs well. At idle, a little rough. So ok, since I was pulling the power steering pump anyways, wanted to check the obvious: Water pump, pulleys, etc. While under there, decided to do a cylinder pressure check (not easy on this car). Got the following readings: Cyl 1: 90, no leak down Cyl 2: 92, barely noticeable leak down Cyl's 3-6 all at 150psi, with minor leakdown noted after 3 minutes.

Ok, ok I thought - it's a 16 year old car - that's not bad. Well, in order to get to the rear firewall cylinders (1-3) I took off the upper plenum. When I pulled the spark plugs, cyl #1 was not even finger tight. ALL? Of the spark plug electrodes looked like diamond tips! I replaced all the spark plugs. However after I replaced all the spark plugs, something hit me as odd. When I was cranking the engine over (yes wire harness to distributor block removed, so no spark) I noticed compressed air coming upwards out of the plenum. This struck me as strange, so I did it a few more times. Very noticeable air forced out of cylinder 1 & 2 plenum intakes. CRAP! Something's amiss. (Pun intended)

What do you think might be the most likely cause of air coming out of the plenum at these two cylinders? Pressure test in these cylinders is low, but no leak-down noted. What do you think is the most likely cause of this?

1) Head gasket?
2) Valve seat?
3) Bent valve?
4) Something else?

I was all ready to put this car back together too. :(