[though im far away from gigging][this rhythm riffs mind you] the same thing happens...any advice? much appreciated
I've never had this myself, but i suggest that when it does happen, movie your fingers and wrist and just stretch it all out. Don't hold so hard on the pick when playing. If you are a beginner, which I am not saying you are, it may just be your body getting used to it. I don't know if this will help, but hey, I can't hurt anything can I?
You are pressuring your hand to much, don't hold your pick for dear life, but hold it tight enough so you don't lose it.
Quote by dudetheman

an abomination. It's like a BC Rich had sex with an Ibanez, had an abortion, and it survived on the side of the road eating roadkill and drinking it's own urine.

Most metal guitar ever. Period.
well if you down pick em all that will tire you out faster, although james htfield has a crazy right hand and down picks almost everything, impressive but not very useful,
if you wear your guitar pretty low, you have a very sharp angle on your wrist that could cause cramps, gripping the pick hard, keep your hand relaxed, or maybe you just need to build up the sendurance
You don't just run a marathon in three hours; you build up to it by running a mile the first day and then collapsing in exhaustion. There may be flaws in your technique, but once you get those worked out, you just need to build up the stamina.

Also, it's fine if you want to downpick everything (within reason, not 16th notes at 180 but maybe 8th notes), but if you need to rest for a minute, feel free to use some upstrokes as well. In a live concert, there are so many other factors that affect sound that a few upstrokes will not be noticed. On a recording, however, you may choose to suck it up and just downpick, as differences there will be noticed.
just a little thing to add: Stopping and stretching those wrist muscles will help a lot too. My favorite is to hold your right hand straight out like your saying "stop!" and gently push your fingers back. You'll feel the bottom of your forearms get nicely stretched if you do it right. And you'll get wrist-stamina quicker/prevent injury. - learned that from working out - feel the burn!!!
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