Don't forget you can make a coolant system pressure tester very easily
and cheaply. I made one for about $15 CND.  It's great for looking for
coolant leaks, or in your case to find out what the heck is going on.
Parts needed are an old coolant pressure cap with guts removed to allow
free flow of air, a air valve stem or nipple (to pressurise the system
with a bike air pump), an air pressure gauge, a short piece of fuel
hose, and a couple of brass fittings and hose clamps to hook everything
together.  P-mail me and I can email you a picture of mine if you like. 

Pascal
'84 Westy

"Also check the coolant system pressure, it should be at 13 pounds.   If
it's more then you have a problem."

Cheers,

Frank  Condelli