Hello
intermittently, there is an open circuit on one or both of your brake lights, in my case this was down to a lamp itself, believe it or not, but as you have changed the bulbs it will just be the yukky process of checking all the connectors.The lamp holders themselves arent actually that clever, I've removed my complete clusters just to fit them correctly, I find them quite awkward.Do you have a good multimeter?, if so start checking the dc resistance, although inductive, at dc your lamps will measure short circuit.I'm not sure how much restance it takes to get an alert, but it probably isnt much.Try to avoid spraying anything with WD40, or anything similar, although a favourite with electricians, it just attracts dirt and causes more problems than it solves at low voltage levels.
It may also be worth checking the 0 volts return line, this is often a nuisence on rear light faults, youve seen the type, indicators flashing ok until they brake, then the whole cluster flashes dimly.
It could actually be what is happening to you.
To clean electrical contacts up, try to get a fibreglass brush, jolly useful things, although wear gloves when you use it, dam stuff hurts like hell if you get a bit stuck in your hand, and you cant always see it to get it out.