Hey guys, I've been playing guitar for quite a while now, I'm far from being good at it, but somehow I managed to play a riff from one of my favourite bands, but now I'm experiencing some trouble, in a riff that repeats itself alot of times throughout the song, ( http://www.youtube.com/watch?v=bWOAWN6kSDA ) it starts at 0:30 , I can do it, no problem, but when I repeat it some times, my right hand starts getting like super tired, and my speed decreases drasticly, and I can't keep up with the song. This is really frustating because I really like this song and I can play every part (solos apart ofc), but my right hand just gives up on me!! :(

Any advices would be very appreciated, thanks ;)