rear light cluster

Hmmm, doesn't look like this will be as straight-forward a fix as neither the plug or the pins look to be in too bad a shape. What makes the diagnosis more difficult is that the stop/tail is controlled by PWM so there is only one filament in the bulb which is made dimmer/brighter by the BCM sending out different signals. If it was a permanent fault, I would suggest that it's either the BCM itself or a software glitch causing the issue but you're saying it doesn't do it all the time.
I myself (being a tight git and not wanting to spend money ) would prolly try to re-create the fault by giving the plug/wiring a good ol' ragging about while getting someone to sit in the car working the brake pedal and see if you can bring the fault out during a particular time but I do understand that it's gonna be a right PITA.
Hope you manage to get sorted buddy, hopefully someone else with a lot more experience than me will be able to offer some further advice/testing.