iPhone 14 Plus restarting or powering off randomly every five minutes

First off the iPhone 14 Plus you purchased couldn’t have been brand new as it is now half a year old from the discontinuation date. Either this iPhone was used, or it’s unused but sitting in a box, which is not good for the iPhone and causes it to degrade over time. I suggest you return and exchange it.

With a Battery Health of 87%, it may be newly bought by you, but it surely isn't a new iPhone. That phone isn't that far off from needing a new battery, which will cost you $100. And there's no telling what other issues the phone has, which would not be covered by warranty at this point. I'd return it to whoever you bought it from and if you want to buy a used iPhone, buy one from Apple's Refurbished Store. It will come with a brand new battery and screen and include a 1 year warranty from Apple.

Certified Refurbished Products - Apple

Also, read this about buying used devices --> The All Too Common SAD Reality of Buying … - Apple Community

iPhone 14 Plus was last sold new in February of 2025.

If you bought it from Apple or an authorized reseller while it was available new or refurb, it’ll have a one year warranty, or longer if AppleCare was purchased.

Id purchased used, seems you bought somebody’s problem. Return it to the seller for a refund, or expect to pay for repairs.