Hi nick, BIS engines LOVE eating head gaskets. I've taken heads off and the old gasket just falls apart between my fingers so I would be suspecting that this has happened to yours, although not as drastic as the one I last did. I think the reason it regains its compression is because as the engine warms up all the components expand due to the heat and it effectively re-seals itself so perhaps a pin hole sized hole has appeared. There are fluids you can use like K-Seal which works like radweld and seals the hole after you've emptied it into your water system but to be honest if a jobs worth doing it's worth doing right so I would opt for getting the head gasket done. It's actually rather easy and well within the relms of a diy-er to do