How can I stop a Sony Ericsson W580i switching itself off, etc?

My wife has got a Sony Ericsson W580i which locks up, switches itself off, sends texts only when it feels like it, etc. I've tried to update the software on it but, doing it through the browser on the phone, but it says there's none available. I can't update it from the website because the download is for PC's and I only have a Mac. Has anyone any ideas because it's driving me mad and so is she!...