Well since i've been playing a lot of metal lately I've noticed my biggest weakness. I can't seen to keep chugging away on the low e in rhythm. I don't know why sometimes I find it hard to keep the rhythm. Also when i'm palm muting and picking really fast I have to curve my index finger in behind the pick so that the pick is 90 degrees to the strings...but when I do this it becomes harder to play notes on the A and D strings.

Lamb of God does this a lot, as someone on this forum described it. "CHUGGA CHUGGA CHUGGA WEEDLE WEEDLE CHUGGA CHUGGA CHUGGA"

How do I go about improving this so it's easily to play?
You keep trying It's something that really just comes from practice (as all things) but there's nothing REALLY specific that can help you other than slowing down and making sure your muscles are adapting to the pick movements. Also, lose the tension in your hand and arm (if you don't have any then you're doing it more right than most people). You just kinda have to "feel" the rhythm and just keep practicing it. Once you get the basic sound down, try to play some songs that use it often. There's not much you can do about it other than just keep trying. It's kind of like pinch harmonics :P At first it seems difficult to get the method down, but once you practice it and just keep trying to do it it eventually becomes fairly natural.