I know that I sound like a broken record, but this is an OS issue. We see 1 or 2 new threads like this a day.
Since the last update, the Droid X seems to have started doing random reboots, even when you are not using the phone.
You can hard reset the phone, clear the cache, format the SD card, etc... There are a lot of people with this problem, and it has never been narrowed down to a particular app. And there does not seem to be a fix, other than using a different ROM, which is a modified OS.
You can try it and see if you get lucky, but I have tried everything short of using a ROM, to no avail.
totally agree in what you have said, Some phone are affected with this issue and others are ok, so far.
I have had reboot issue in the past to a minor state. However seeing that I Flash Rom's and SBF about twice a month plus use the latest kernel version at this time my phone runs great.
Is there a simple answer, no.
Are there ways to try to fix your issue yes but which one depends on your OS,setting,and installed program.
Yes you may just have a lemon phone .I've read of people having to go through more than one replacement to get a phone that operates the way they expect.
EDIT: Try a Rom....You just may surprise yourself LOL