Ok I've been playing guitar for three years. I got my electric just last year. I've been trying to play alot of metal, mostly Metallica. Im a natural down picker but, when I try to play something like Master of Puppets (the part after the intro) my picking had starts to slow down... alot. I have tried slowing it down and working my way up. I can play along at about 180 bpm but, if I go faster that 185, I start to slow down alot. and then I can't play even at 170. Do you know what I can do?
practice longer. it sounds like you're fatiguing yourself around 185, like a sprinter burning out after 100 meters. gotta train for that marathon man. there was an article on ug a while back titled "practice the downstroke til it gets boring" or something like that
I've improved mine a lot by getting the "Blackened" guitar pro one, slowing it to about 150 bpm and just playing it over and over and over until I get really bored. This has been going on about a week and while I know I won't get good in a week, it definitely helps. Guitar Pro is your friend!
Just keep playing that part at the same speed for a week or two. You WILL see an improvement.