Haven't had much to do with this kind of thing but i know the factory gauges and sensors can be pretty average, do you know anyone with an aftermarket sensor and gauge to see what sort of temp your running.
If you think it as a whole, the water pump always is running, the radiator is new and so is the coolant, and the only thing that normally stops your temp continue to rise is your thermostat, providing your thermofan is working.
So u would say that maybe your thermostat is sticking, like it pops but when it closes it sticks.
or as limbo said about the bubbles, but you did try that and it stopped for abit then started so maybe you have cracked hoses?
as I said not really touched on this before but just abit of food for thought