Make sure you are holding the pick correctly. Generally players hold picks between the thumb and the index finger. Ensure that only the tip of the pick sticks out (ideally 2mm or less). That will prevent the pick from 'flapping'. Also try and make sure that your thumb joint is as flat as possible i.e. you are not bending your thumb outwards. Any tension in your picking hand reflects in your playing.
As for your question on how tightly to hold the pick: You should hold the pick just tight enough that it doesn't slip out of your hand.
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