iPhone shudders

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!

You need hold them down long enough for the screen to go black and leave them held down until the Apple logo appears. Make sure you press these buttons together, simultaneously.