Is this in any way connected to a holster/case with a magnet in it? The X doesn't like magnets and, if this is what's happening, you need to get a different holster. \
Magnets will activate the Car Dock or Multi-Media modes, which might keep your screens on for a brief while after you put it into the holster/case. Not good, because will drain battery and magnets will "confuse" the X when you go to use it.
Another possibility is the automatic update feature of the Market for the apps you have downloaded. If an automatic update occurs in the background while you are doing something else with the X, you might very well end up with a bricked X. If this happens, turn off/on or do a battery pull.
Best advice: in Market, go into each of your downloaded apps and UNclick the "allow automatic update" box.