HTC One — HTC ONE M7 - Heat issue? or Faulty Battery? Weird - MAJOR crashing issue

Hi, i hope you can help with my issue with my m7 phone.

I have an issue with my phone randomly crashing and shutting down all of a sudden.

For example i may charge it fully to 100% and use it like normal.... then if i decide to watch say a 1 hour video..... it suddenly after some time (varies 30-90 mins) while it is playing it shuts down and goes black, and is not a normal shutdown.

When i try and press the power button it does not switch on... i then plug in and see all charge has been lost and it is at 0-3%??

I really want to get to the bottom of this issue, and hope you could help?

Do you think if i get the battery replaced with a new one it will fix this?
Or is the issue with the motherbaord/cpu/other..... which basically means my phone is not worth repairing and i should bin it?

I hope someone can help as this is a relatively new phone so should have no issues.

Thanks in advance