Hi all
Maybe this is a little bit cruel, and not-professional approach, but I'll tell you a story about my old 6600. I had exaclty the same problem, with my phone shutting down until... I dropped it on the floor.
Since that time it has been working perfectly 
(However I am not telling you to throw your Nokia on the floor to fix the problem.)
Personally I think, that it may be an issue with connectors on battery.
Good luck
hi everyone i had a problem with my nokia 5800 it just kept switching it`s self off then i was on the web trying to find what i could do and found someone else was having the same problem and they said it was a battery concerting problem and that the battery was wobbling about in the housing .they found by putting two rolled up pieces of paper ( like a roll up) either side of the battery so the battery is wedged in. I have done this to my phone and so far so good problem is sorted . i hope this works for others .
kind regards g8mum
I had exactly the same issue with my nokia express music 5800.
I was advised to buy a new battery but it was still switching off randomly.
I noticed the phone switched off when i slightly twisted it, which led me to think that the battery was loosing contact with the 3 terminals.
TO SOLVE THIS: i stuck 4 layers of selotape on the underside (hidden side) of the battery. This selotape was stuck only at the contact end of the underside. This spaced the battery outwards and creates more pressure on the 3 terminals and therefore there was more consistant contact.
Hope this fixes your problems or anyone else with the same issue!
