TS, I guess you have to check your phone thoroughly coz AFAIK, Nokia 8XXX has a lot of problem with hardware, e.g : the battery no problem but sometimes phone shows battery full, suddenly only 1 bar, suddenly phone off itself.

Not to blame anyone or put the seller into trouble, but it's just what I heard. You may try using your frens battery (if any1 using same battery model) & see for some time whether it's really battery or phone that is problematic