Sorry for you pain, Flagz. You mention that you've had four 925s and that your fiancee's 810 has the same problem so if I'm understanding you, it sounds like all the phones are having the same problem. Could it be something in the network? I have no idea how much the hardware is the same between the 810 and 925, and if there is some overlap, it could be the firmware, but I would assume you and your fiancee are in the same spot most of the time, using the same towers on the network. You mentioned that you used to have an 8X - I'm assuming that was fine and you used it in the same location? Four lemons seems like a lot, so I'm trying to think outside the box a bit and brainstorm other possibilities.
I'd say SIM card, but you and your fiancee obviously each have your own.
I'm hoping they'll release the update soon and you can try that.
Are you running the developer preview of GDR3? I've had my phone for about a week and installed it day one with no issues. Doesn't seem like it'd be an OS issue or others would be having the same problems.