Improving USX form

Would love to get some feedback. I’m running into two problems: 1. I’m starting to get pain in my elbow anytime I do a speed building practice session lasting longer than 15/20minutes. I think it’s because I tend to tighten my grip to keep the pick from flopping around, which creates tension up my forearm and elbow. Is there a good way to maintain a relaxed pick grip and still have control at higher speeds? 2. Im hitting a speed limit with EJ style pentatonic runs at about 95 bpm 16th note triplets. Looking at the tape, I think at some point during these runs I make a switch from the straight diagonal USX motion to something sloppier with more rotation in it. I think I’m capable of the USX motion on single strings (see the 6 NPS video… this looks and feels a bit cleaner to me) but something breaks down as I switch strings. Any tips would be greatly appreciated. Thank you! Daniel