help on riffs!

im having a hard time playing fast riffs like the master of puppets by metallica. everytime i do it fast my right hand always seem to get stiffer and stiffer till it reach the point where i cant move it any more. are there any techniques in nailing that song 100% or just practice? I didnt know downstrokes can be a pain in the ass.. =X so help!

Everything is about timing, and all that is about practicing :) Does'nt matter how fast you are in your left and/or right hand if you can time them perfectly with each other. But if you start slow and go faster and practice with a metronome. You will notice the difference.
When you gets faster, you'll feel slower for a while and then you will notice that you have been faster.

Chill tAUG