cracked block?

[/<eystroke]

A couple of months ago I started loosing coolant while the engine was running.
It is up to a gallon every 150 miles. I never found a leak that would make me
loose as much as that. So I kept looking for leaks and trying to fix it with
barrs stop leak. I finally decided it was going out the tail pipes even though
it doesn't smoke. There is pressure building up in the cooling system when
running that will stay over night. This makes me think it isn't from heat
expansion but from cylinder pressures. I've replaced the radiator and the cap,
I've tested the heads for cracks and warpage, I installed new head gaskets and
still have the same problem. The cylinders walls were in very nice condition.
I didn't see any cracks or holes in them when I had the heads off, but I guess
I could have missed something. The spark plugs don't tell me anything. They are
all slightly sooty brown.

I'm not a great mechanic, but what else could it be, other than a cracked
block? There is a miss at idle, but I always thought it was from my defective
distributor.

Anyone have any ideas?