I've done some wrist exercises that help loosen things up. Play every other white note, but use your wrist to control the notes, not fingers.
Try this -- very,very,very slowly move your fingers into and out of a fist. Take a deep breath and relax all the muscles in your hand. Or, if you're tensing up the muscles only tense that ones absolutely necessary to move -- let the others relax. When you release a deep breath, it's easier to relax.
I've had pains that took about 6 months to go away. The ultimate solution was to slowly work up to that level. I was pushing too hard, too fast. After 2-3 days of hard playing, my hands felt stiff and I started regressing and got a lot less accurate.