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!
My stuff

Gibson Les Paul Studio
Ibanez ADC120
Tanglewood TGRF VS
Blackstar HT20
Roland Micro Cube
Just keep playing that part at the same speed for a week or two. You WILL see an improvement.