Update Sept. 30, 2011: With the Droid-X continuing to crash under these fixes, a new solution emerges: simply remember to hit the home button before locking the screen. It seems that the rotation of Pandora (and other apps like DoggCatcher) that occurs while locking your screen may be responsible for most of the crashes.
Update March 7, 2011: It seems that uninstalling and reinstalling the Gmail and Pandora apps might help, although it’s not yet known whether the order matters.
If you own a Motorola Droid-X, you might be dealing with semi-random crashes when using the preinstalled Gmail app, where the phone reboots when you try to send email. For me, the usual “clear the cache” type tricks did not work. Neither did setting the phone back to the original factory state.
The solution: go to the Market and install the Gmail app from there. You’ll see the app changes in appearance and behavior a bit. This is probably the easiest way to resolve the crash / restart issue. Surprisingly, there was no “update notification” for Gmail – I just tried going to the market on a whim and discovered the update.