Sleep/wake button not responding while charging

Hi

after iOS9 update my phone started to act strange. Next morning after update (I did it right before sleep) my phones Sleep/Wake button started to give out. It wasn't responding to sleep or wake commands.

After some digging I figured out how to restart phone without S/W button (went through settings to reset Network Settings which restarts phone) and after restart S/W button was working as normal....

...for 45 minutes!

Same day I decided to reset my phone through DFU and set up as a New iPhone... all went well. Reset and set up again and managed to instal all apps, all good.

then before going to sleep I set up my alarm and plunged in to charger and SLEEP?WAKE BUTTON DOESNT WORK AGAIN!

button works when phone is unplugged but doesn't respond when plugged in to charge.

I'm using all apple original chargers and cables.

any idea anyone? iOS9 or hardware?