I have this same problem every once in a while (but only my left brake light is out). I've gone through 3 bulbs , only 1 of which was actaully bad. The way the connection is set-up give the bulb a bit of wiggle room, so sometime the bulb doesn't always stay pluged-in.

Last time it happened I was so pissed I went to my local mechanic and told him about the problem and asked him to take a look, he said my bulb was bad and fixed it.. haha. No problems since.

Maybe explain the problem to a local mechanic and they can fix it for you so that somehow the bulb will actaully stay connected.. as it should...