Hmm.. i think i get what your saying..

I initially thought bad contact i.e earth points around cluster of light or at contact of the brake pedal switch - causiing a varying/fluctuating amounts of current to go to the bulbs - perhaps?

You could also check that both of the filament wires are intact in the bulbs.

Sounds pretty basic and you may have done all this stuff already - but give it a go if you haven't, might get lucky.