If you swapped cards with a mate, you will find it'll work.
Did you say the video card temperatures actually dropped?
"Before, it was 70ºC for the video card, now it is 47ºC"
So it can't be because of heat issues, but I wonder why its suddenly noticing such a drop. more cooling?
Plus when you took that first temperature, it might have been playing a game? Whilst on the second reading, it wasn't?
Plus even though you un-installed drivers for the card, the card was still being used after you restarted, yet with VGA drivers which are ones that are used with Windows when you first boot up with a new card, or in safemode.
The fact that its appearing at bootup and not in desktop in this mode confuses me...
I seriously think you've done some damage mate
Also the fact it says overclock failed...is it stuck at some point? Or by any chance have you installed a bios update for your video card without knowing to make it better than it is? (There are bios updates that will fake a card to think its something higher and run it at that speed) so maybe something like that? So its actually running at a too high memory and clock speed when you start up.
I would seriously consider getting a new card. Something like a Geforce 6600GT would do nicely (about £160).
I hope this helps