BB Bold :: Faulty 9780 - Keeps Turning Off?

In short - i purchased a 2nd user bold 9780 off eBay back in April / May timeThe phone was like new and worked fine until recently the thing began turning itself off.There is no pattern to it - its very irregular which makes it even more annoying.Essentially i will be writing a SMS or BBM and it will turn off. I then have to take the battery out and reboot the phone. This can happen once a day, or every couple of minutes. Now .... i believe the phone is still in warranty so i went to the vendor who originally sold the phone to the person i bought it off (Virgin Media) I know this as it's their branding all over the phone / box.

Firstly Virgin where reluctant to help me as i wasn't the person who originally purchased the phone. This i didn't buy as, at the end of the the phone is still in warranty. After the half witt behind made to enquires and checked the serial number of the phone against there records - it showed that the previous owners owes Virgin Media money on a outstanding bill .