I recently purchased an Xperia ray (ST18i) (parallel imported from Fair Buy), and I'm having reception issues. 3G is very low signal and hard to get going. Since I don't use mobile broadband, that's not too much of a bother for me at this point, but it also seems that 2G is flaky. It often disconnects randomly, leaving me to go into the settings and manually try reconnecting (which almost always works). The odd thing is sometimes when it's "disconnected", it shows Vodafone as an option in the cell sites setting, and it's selected (ticked), and 2degrees is there too. Of course, if I try doing that manually, it doesn't work. Maybe that's just an unrelated display bug. So what can I try? I've messed around with the settings (manual/automatic site network select, roaming/no roaming, etc) to no avail.

I should also note that I'm using the same sim card I had in my old phone (a Sony Ericsson featurephone), and that had no such issues. Also, this is stock firmware, fully upgraded.

Re-edit: I'm in Wellington, phone network specs are

2G: GSM 850 / 900 / 1800 / 1900 
3G: HSDPA 900 / 2100