boot loop

Hello to all, I have a new to me iPhone 8 that wont stop rebooting. Had it taken to a shop and they replaced the IC part. Issue is not resolved. After they kept it for 4 days and said they didnt see any issue? I have it now and there is definitely an issue! The phone is barely with a scratch...

I recently bought a refurbished iPhone 6 and its been working perfectly for 2 weeks then it suddenly started a boot loop where it wouldn't go past apple logo. once or twice 50 times it would come to lock screen and even past that but it would last only couple of minutes. then it would continue...