2000 Ford Taurus

I have a 2000 Taurus 3.0 single cam with 120000 miles on it. It had a coolant leak at the timing cover and I kept on adding water. The car was left out in 15 degree weather and my son drove it the next morning until it stopped running. It overheated but started after it cooled. The lifters were collapsed and it made so much noise I had it towed to my home garage. I drained the oil and there was little to no coolant in the pan. There was however white foam in the valve covers and air intake tube. I pulled the heads off and had them cleaned, magnafluxed and plained. I put it back together with a complete head gasket set, new lifters, head bolts, timing cover gasket and water pump. It ran great for about an hour then started to loose coolant and then skip at idle. It also made a loud groan in the air cleaner box. This time it took in about a quart of coolant in the oil. I had a gararge do a compression test. The back cylinder on the radiator side of the engine had 20 pounds less compression less than the other 5. I tore that head off and brought it to the same machine shop. They checked the head and said it was alright and it looked liked the head gasket sealed correctly. I put it back together a with a new head and intake gasket. It ran at just below the temp gage midpoint and the coolant level didn't move. Again it ran beautiful for an hour and then started to take in coolant and skip. I'm really frustrated at this point because I can't figure out whats wrong. Here's some thoughts; Could the cylinder wall be cracked or maybe the cooant passages around that cylinder be plugged? Am I torquing the head bolts incorrectly? Is the coolant coming in from the intake manifold because I hear a groan in the air filter box? Your help is appreciated.
April 22, 2007.

Could be a cracked block. Water jacket, Do a pressure test.

Apr 23, 2007.