welcome to the forums, this thread is meant for the touch pro but the diamond is pretty similar, you may have better luck getting help in the
diamond specific thread for this rom found here. to me it sounds like a setting within tomtom 7, there should be something along the lines of keep alive backlight to leave it on, are you having the same sleep problem with any other applications like tcpmp while playing videos? also do you have your device plugged into charger while running tomtom?
here is a
link to the titanium only energyROM for diamond, more choices
here too