What do you mean by shudders?
Have you tried a hard reset of your iPhone by holding down both the Home and sleep/wake buttons simultaneously until your iPhone goes to black and restarts with Apple logo, then release the buttons?
If that doesn’t work, since your iPhone 8 is new, you have 14 days to return it for another iPhone 8, return it to your “local” Apple Store for a refund and/or another new iPhone 8, AND, if you can, test this other new iPhone 8 in the store before you leave.
Also, make sure to purchase 2-year extended AppleCare+ for your iPhone 8 within 60 days of purchase.
I’m going to have the ASC moderators move your post to the iPhone Communities.
Good Luck to You!