cracked block maybe, pressure has to come from somewhere a sniff test will tell you whats going. thermostat is clutching at straws as it would still run ok regardless. bursting a heater matrix and blowing off hoses is a major problem a leak down test would be an idea along with a compression test for a more definate answer. a quick look at the spark plugs should show if it's burning water one will be cleaner to show the problem on that cylinder