I've had a similar problem with my 43 MW, she needed a new water pump and during the fitting of this I found she had no thermostat, but I've not had her on a run since. She usually "foams at the nose" after a short burst of power after she's hot, but post fitting the new thermostat she seems to run (in the garage!) a lot better. I too did the compressions and got a consistent 140 psi on each pot, and there's no oil in water or water in oil! someone said to me that i could have overfilled her with water, but after a run when she's foamed the water level is below the fin tops on the rad and I'm not sure that that is correct! I'll give her a run friday and see what happens, and let you know. I may fit a water temp gauge and have it hidden somewhere, though I would like to keep her original, I don't want to toast a good engine by thinking she's just blowing a bit of excess water when she's got a real overheating problem!
I'll be back
mike