well, if a gear is missing a tooth or something then when you hit the brakes it stopps the diff, then if your engine has too high of a idle it will keep clutch engaged, which will make your gear mesh jam momentarily, that will cause your engine to stall

so my suggestion is to just richen the engine up a little, or if it is tuned ok then lower the idle while the engine is running, if that aint it then it has something to do with the drivelines

AND MAKE SURE your servo isnt holding the throttle open any, then when you hit the brakes it kills out the engine, turn all your radio stuff on and then look in the carb, hit the brakes, if the carb moves then its set up wrong