i think it sounds like a dodgy switch, i had the same problem with mine but fortunately i was lucky with my ebay switch

Bridging means you can make the circuit with the switch not there, you could unplug the switch and use a bit of wire on the loom to link the two terminals together. this should turn your brakelights off. once you have done this and the lights stop being intermitant you will know it's the switch.

Hope i've worded this ok for you :P