This symptoms may be related also to a known bugs in RDR-HXD series DVD recorder. 

When you say that the channels "later vanish" you mean when you power off the device? If this is the case try the following:

• be sure you have the last firmware installed (1.70)
• hard reset the recorder, pushing stop and poweroff buttons together on the deck (be sure no disk is in the tray)
• unplug from main for 1 minute
• restart and follow the easy setup procedure including the rescanning of the digital channels: once they are found they will remain (at least for some month....)

Resetting the recorder you'll lose any custom setting you made (i.e. HDMI video setting) and any timer. Nothing will be changed in the HDD (so your recorded titles will stay there).

Hope it solves