I'd be very interested!
An update on my activities: haven't gone to AT&T store to get replacement sim yet--may not have time until next week. Also, phone has not reboot/shut-off since I set it to GSM mode only. Also, I found someone else who had the same problem (mostly shutoff) on E63 that started within last few months--he "fixed" it by using only GSM mode. I will ask him about his version of firmware.
I'm kind of losing hope that a replacement sim will fix problem--especially since you've already tried it. I think the new firmware will be best bet. I will do this as well--but probably won't get around to it until at least next week. I will update when I do.