OK, that's weird then. I don't think we have heard of this exact problem before, and will only be able to suggest ideas.
- Ensure there are absolutely no possible sources of interfering IR radiation;
- Reboot the handset and reset the control channel number;
- Warranty return - it has been known for the IR receiver module to malfunction;
- Reload the firmware;
- Use custom firmware to check the file system for errors (and fix them);
- Use standard menus to reformat the HDD (destructive of your recordings);
Beyond those you have already tried, I can't think of any other options (in a non-custom firmware environment).
If you are a custom firmware user, make sure you have not set up the ir
package to cream off the button presses, even accidentally, and use the virtual remote control to inject the button presses to see if they work that way. It is also possible to monitor the incoming commands.