iphone slide to power off frequently

Sounds like a problem with the Power/Hold Button - maybe is stuck or gets pressed too easily. I would take it to your local Apple store and get them to check it out.

Have you tried the basics from the manual?

Restart.

Reset.

Restore.

Strangely, I have the exact same problem, it started yesterday too. I've reinstalled the firmware and it was doing the same thing before I even restored from backup.

No warranty and not even an Apple Service Centre here 😟

yes i have done the reset and restart still am facing same issue.

What about restoring in iTunes as already suggested? Restore first using a backup and if needed, as a new iPhone without using a backup. If that doesn't work, you have a hardware problem.