I just stumbled across this thread and am having what seems to be the exact same problem. My issue seemed to start back in March and has not improved.
I had two Aria phones. One stock and one running CM7.1. They both developed the random reboot issue. I have restored them to stock and they still randomly reboot. With and without SIM or SD card.
I finally broke down and purchased a new HTC Aria which arrived yesterday. Stock... with Android 2.1 just like it came from AT&T back in 2010. And, sure enough, random reboots!
I have no clue what is causing this. I have contacted HTC and they suggest that I get a replacement for the new phone to see what "happens" and if the random reboots occur on the "replacement" of the replacement, I can send it to them under warranty but they did caution that the issue might be the cell network. Changes in the network could result in hardware incompatibilities that would require firmware updates which they are not planning on providing. To test that theory though, the phone would have to be used in an area where another Aria phone is successfully being used.
I talked to AT&T and they claim there have been no hardware upgrades or changes in my area in the past three months. However, I know this not to be true seeing that I have an LTE device which just recently started functioning in LTE mode.
Bottom line, I am at a loss for what could be wrong. I find drumist's idea of the SD card causing hardware damage to the phone to be interesting. That is one consistency here. I too have used the original SD card in all three devices as I moved data around (backup/restore/roms). The thought of an SD card damaging a device seems scary.