I have been working on getting faster down stroke speed. and I found that when I get to faster speeds my elbow is doing most of the work. how do I get out of this habit?
Well, in order to gain speed, you have to relax your arm/hand (which is hard of course, because at the same time you're instinctively concentrating on putting more and more power to it in order to achieve speed). I know taht if I don't relax my arm and go downpicking fast, after a couple of seconds my hand will feel tired and I'll lose a great deal of speed.

Easy in theory, hard in practice. Try to be as relaxed as possible when playing (especially when playing fast parts). Let your goal be to play relaxed instead of playing fast. Concentrate only on being relaxed, let go the speed for a moment. I know I find myself downpicking fastest when not concentrating on speed, but concentrating on melody for example. If you feel your hand is getting tired while playing, you're doing it wrong. Stop and give it another go. It's more a mental thing really, you have to beat that barrier in your head and the body will follow.