The phone is rebooting every 5 minutes.

Hi!

I bought an iPhone 5s on eBay. The seller said the phone is randomly rebooting since the iOS 11 Update, so I thought it is just a bad update and I restored it on iTunes. But its just the same as before. I've got the same symptoms as you all. The phone is rebooting every 5 minutes. The seller also said that the battery is also replaced. So I changed it. But it didn't get better. I also saw, that the screen issn't an original one. Do you guys think a replace could help it?

I spent about $60 on this phone and I dont want to throw it away.