Sorry to start with bad news, but to me it still sounds like your head gasket or another route is pushing combustion gases into the cooling jacket. This sudden pressure can push the water out, causing the expansion bottle to fill and your heater run cold (no water in it). I am assuming this bad news as you heard the "pop" first, then the temperature went up, suggesting it was over pressure before temperature.
I would pull the plugs and look for coolant in the cylinders. Also drop the oil and see if it's making its way into the oil.
A stuck thermostat won't help, but more likely to just overheat initially rather than blow the coolant out. But if it overheats enough you get head problems.
Check the plugs / cylinders for coolant.
Check the oil for coolant
Check the thermostat.
Check the radiator flows - IE it's not blocked.
If you suspect thermostat change it and try again, nothing to loose, it will work or it won't at this stage. Hope it's just the thermostat or block radiator and I am completely wrong with my assumptions.
Mike
Yokohama Geolanders, Sony head unit, NAUTILUS Air Horn, DRL conversion, Rear cargo space, Elvis Bobblehead, transfer Guard, Indian hanging Elephant, Koni Heavy track dampers, Custom SS exhaust, Voodoo Doll, Adventure Rack with LED ight bar, vintage ERIBA caravan usually attached (yes it's slow)
