N900 Won't switch on unless charging
 

I've had my N900 for around 6 months now. I only noticed around 3 weeks ago that it will not switch on unless it's plugged into a charger or a computer, even if the battery is fully charged.

When it's not plugged in to anything:
> Press power button.
> LED does nothing
> Nokia Logo appears but no backlight
> LED slowly fades yellow (Like when charging) twice then one fast green flash and the phone switches off completely.

When it's plugged into charger:
> LED slowly flashes yellow
> Press power button
> LED fades in white once
> Nokia logo appears with backlight
> Logo dissapears but screen still on
> Nokia USB Logo appears
> 5 dot logo appears with back light
> Backlight switches off then on again and phone boots up.

I've tried reflashing and also reflashing + emmc reflash.

Completely lost.

Anyone any ideas?