has you check to see if you have air in the system after getting things done it sounds like you had a leak and a pretty bad one at that if you were adding fluid every few miles and if you thermostate failed it would keep cool not over heat its a failsafe with modern thermostats they fail open not closed well in most cases that is also how is the water pump with over 100k the think could have just died those things don't last forever and that would give you a good reason for the loss of fluid and over heating as well