My W800i has been acting strange lately. For no particular reason it refuses to play mp3 files, the themes are messed up and if I try to write/send an SMS it turns off. BUT it happens at random and only once a day, when I turn it off and on again everything's ok. Seems like memory overload or something like that to me, but I'm not sure. Has anybody run into similar problem?

Try updating the firmware, just goto Sony Ericsson site and download the newest version.

I have my w800i since March of 2006 and i have never faced out a problem like yours.
I think you should try to do what dezal said