[Capri306]

I'm willing to bet his system overheated (or started to), popped a small hole in the heater core, and eventually built up enough pressure to start bubbling out of the radiator cap (which might have stuck shut).

Here's how I think things will go:
1. You're going to have to replace that heater core. Sorry, but that's how it goes on these cars.
2. Replace the thermostat AND THE RADIATOR CAP at the same time, as it's easier to mess with the cooling system once instead of cleaning up coolant twice. The thermostat for obvious reasons, and the radiator cap because it's a regular maintenance item (it has rubber seals and a spring, so it qualifies for a replacement).