Apologies for the delay in responding, my friend. If you're still having issues:
As of the latest firmware, there's an "AutoOff" feature added to the DTXM12 that will power the unit off if unused for more than 30mins (30mins is the default, but can be adjusted). You can adjust this parameter by going to UTILITY -> GENERAL -> Scrolling right to reach UTIL1-9 -> Use the [-] button to change the setting to "Off."
Hope that helps, and have a great day!