Palm muting troubles!!?

O.K. , Im in a metal band and we play some fast chugging songs and for some reason i just cant keep consistant chugs, it seems like i get 3 or 4 and i start skipping em'. i have been doing excercises and such but it always comes back to haunt me. my brain knows the rythm but my hands just say **** it! ive noticed when i played metallica when i first started playing i was always on key, now i just cant keep the chugs going? IS THERE ANYTHING I CAN DO TO HELP BETTER THIS?