Its good to hear that you also solved your Standby of Death problem but by changing SIM, this doesn't surprise me, did you also change provider or was it just a faulty sim?
I don't think a change of SIM would benefit me as I had the problem with my old SIM (I don't think my SIM is faulty per se not if they are all like that) besides changing the band setting from auto to either 3G only or 2G only works for me and I'm happy with that. If I didn't have a working solution I would def try changing SIM however and I don't want to discourage doing that.
Also I would only get the Standby of Death 2-3 times a week which coinceded with being in a weak 3G area (where the phone might auto switch bands) it didn't happen as often as you had it. The problem is def connected to the SIM card.
Obviously everything I've written above only applies to those who have the Standby of Death because of the SIM or settings which impact the SIM, to rule out a software cause are best to start with a hard reset.