Help me please! Problem with SE K800i

It's my first time posting here, so if I am posting in the wrong forum or if this problem has already been dealt with, please kindly correct me :)

I recently got myself a SE K800i phone. And I got it from a friend who just came home from UK - that would mean taking it to service would be pretty much impossible. Also, I tried SEUS but even that doesn't help. It said that my phone is already up to date.
Things seem to be fine the first day I used it. But then, I started receiving these problems:
- Sometimes when I charge my phone, it would say something like: Charging Error. Stop Charging now! (This often happens when I receive a call while charging)
- Sometimes the phone would just malfunction at screensaver mode - basically meaning that it just stops working and I'd have to restart the phone to be able to use it.

Please let me know if there is a solution to this, and I would like to know if flashing the firmware would solve this problem.

Please, I'm in a pretty desperate situation. Any help/suggestions would be much appreciated. Thank you

If is a hardware problem flashing could be risky, do you have any idea which CID is your phone?

It's CID49
(Oh how I'd waited to get a reply :D)
Please help me. My phone hasn't improved at all, and I've tried all the basic solutions too (Master Reset, SEUS...)

So y dnt u take ur phone to a SE service center?

I did. But the people there said that the phone wasn't original, plus there wasn't any warranty (I got this phone from a friend - and he got it in the UK, see :) )
That's why I'm so desperate for solutions