With the coolant reservior very close to full, the coolant only has to be pulled up about an inch or 2 to get back into the rad, so for sure there is a rad cap issue.
Here's a few more thoughts/symptoms: When the temp gauge was really high, you could hear the sound of boiling in the engine block (or it sure sounded like it). I was able to intermittently get heat from the interior heater, and the hose on the right side of the rad was hot (normal kind of hot, not super hot). If it was a thermostat issue, would the rad hose on the right side of the rad at the top be hot, or would it stay cool until the thermostat opens?
The car has an unknown mileage on it since the speedo and odometer have been intermittend working for maybe 1/3 of the time for the last 3 or 4 years. It's for sure over 300,000 km, close to 200,00 miles. Leaking gas tank when full, dead A/C, dead rear wiper, oil drips, a bit of rust at the back corners of the rear wheel wells. Not much in the way of new expensive work done on it recently, so it might be time to cut my losses if this is a head gasket. But, it's been 100% reliable, never a breakdown or non-start ever, so can't complain.
Thanks,
Peter