I'd go with the water theory too. I suspect you have a small coolant leak that only shows up when hot. Do you have a pressure tester for your cooling system? If not, buy one. Best money you'll ever spend if oyu have old vehicles.
Remove the radiator cap, and drive the thing until it's
warmed up. Yes, it'll steam a little, but it's OK. Once it's hot, park and use the pressure tester to see if it holds pressure. I bet you will find it leaks down slowly.
If it does, you *might* get away with tightening the head bolts, but I fear you will need to pull the head and chekc for cracks.
Good luck!
George
