You can rule out a bubble in the cooling system by drilling a 1/8" hole in thermostat
That way, when you run the engine, any air will bleed of fairly quickly.
There should not be ANY bubbles circulating in the system when the water is full and it is at operating temperature. None.
Any bubbles, particularly a steady stream of little tiny bubbles indicate a leaking gasket. Not blown, just leaking.
I think your spark plug for #5 looks a little too clean compared to the others. That would be where I would look first.
Good luck!
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