I got this phone (foolishly

), and I agree, its absolute pants (

)
Mine (had it for about 3 months too) randomly turns itself off, I assumed this was because I have too much music on it (which is causing it to overload (~2GiGs worth)) as the music player is always locking up (and the first time it rebooted was while listening to music).
However, speaking to SE Repair Centre, it is common (all too common), and the only way to fix it, is to send it to them (as mentioned by prkink ^^). I've personally given up on the phone, I have no use for it now, other then as an alarm clock (

). However, if you do decide to get it replaced by SE Repair Centre, go through your service provider (if you are on contract), as at times (my experience last year), they can take a while to repair the handset, and get it sent back (~5 weeks

), so going via your provider means (providing they are also aware of the fault) may be able to arrange a cover handset, while yours is being repaired.