iPhone 6 behaving like power button stuck on

Hi, I have changed the screen on an iphone 6 and all seemed fine. However it now has a problem which looks like the power button is stuck on but it isn't. It will sometimes fully boot with no problems but then intermittently the power off slider will appear. I can cancel this and get back to the home screen but it will keep happening. I can also use the slider to power down the phone, but once it's off the phone reboots itself and then stays in a bootloop. If I plug the charger in it fully boots again but still switches to the off slider now and then.

If I disconnect the battery and reconnect, I can use the power button normally to switch on the phone and it will fully boot, but once the slider appears the power button no longer does anything.

It's like the power button is randomly activating for some reason. I have checked the physical power button mechanism and it doesn't seem to be stuck or clogged with debris or anything, and the volume/mute buttons are never affected so I'm sure there's nothing wrong with the flex. I have also replaced the battery and the screen again but still the same problem.

Any suggestions gratefully received.