I think I have the same exact problem, and I believe it's head gasket or cracked head...Started mis-firing really bad. ODB gave a code of 0306 = #6 misfire. Removed plug...it was clean and dry and no smell of anything on it. Replaced coil ($120 !)and still no improvement.

Further clues leading to head gasket/head: smell of antifreeze after it's been running. Occasionally the overflow tank will bubble after it is shut off (but never seems to use or give antifreeze). Pressure check of radiator/cooling system showed the pressure while engine was on, as 16psi and climbing. Since it's a 15 or 16 lb radiator cap, that would indicate to me, that something other than heat, is pushing the coolant out towards the cap.

So, 1) how much of a pita is changing the head gasket out? (I'm a chevy guy) and 2) what are the odds it's more than just the gasket - ie crack in head or worse, crack in block or distortion in block? I really don't want to pull the whole motor out, as it's February here in Virginia and dreadfully cold and I don't have heat in my garage

I guess I will pull both heads, and have a machine shop go over them for cracks and warpage and if they check out, put them back on. Ugh...

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2000 XLT 4.0 L Auto Ext. Cab 98k miles