So, I changed the thermostat this afternoon and before doing so had drained about a half gallon or so of fluid. After replacing the thermostat, which looked in good condition except the jiggle valve was at 3:00, I refilled and burped the system. It took more than the half gallon I drained, about 3/4 of a gallon , and the heater now works as advertised. I had tried the burp procedure twice already but there must have been some massive air bubble in the system to take more coolant than I drained out. Thanks for the thread.