Ok JF42
Now you have said something!
How it works is like this. When water gets hot it expands and goes into the overflow bottle. Then when cooling the water in the catch bottle will return to the engine and radiator...
Now if when hot the water in the catch bottle overflows and boiles... you have here a problem. MOST likely you have a cracked head. There is prolly a 15% chance (my opinion) you have a leaking head gasket.
So what is it? Is it normal expansion and contraction of the liquid? OR is it abmormal and being forced out by the compression leaking into the water jacket.
A possible check is to pull the plugs and look for RUST on one of them..
If you are truly loseing water in the manner described you have a problem.. no matter how good the engine runs.
Big Jim