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Sounds like you have a good head for problem solving...you already did exactly what I would have recommended first: slowing it down and analyzing your playing for potential problem spots. I would suggest two things

1) Play as slow as you can and really concentrate on keeping that pick from twisting. Don't worry about fretting or speed or anything else; just focus on what your pick is doing.

2) When you're picking, make sure that the motion is coming from your wrist (and maybe even a little bit from the elbow). Finger movement can be used in slower, more nuanced playing styles (eg: blues) to do exactly what you described: make scratchy/whiny/plucky sounds...good when you want it; bad when you're trying to learn.

Oh, and maybe one more thing. I'm guessing you ment 90 degree angle and that was just a typo...but just in case you're actually using a 45 degree picking angle, that's probably not where you want it...90 or something close to it is what I have always heard (and found works best). Hope some part of this is helpful. Best of Luck!

Are you holding the pick between your index finger and thumb? It is a lot easier to hit the strings a 90 degrees this way. If you have been holding it differently it might take a short while to feel natural.

i use to have the same problem, i never could get it right using just my index finger and thumb. In the end holding my pick with index thumb and middle finger did the trick.

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