What? Are you sure it isnt VHF mode? lol
I think you mean its dropping out of RF mode into IR?
Theres been a few different fixes for that...
for me, it was doing a global reset on the remote and then re-pairing.
others have gotten a new remote or a new Roamio as a fix.
To perform a global reset
The Global Reset function unpairs the Roamio remote from the paired DVR, then clears any other remote settings, such as IR codes for your television or other A/V device. Use this function to unpair a remote and return it to the out-of-box settings.
IMPORTANT: You will need both the paired Roamio remote and the DVR to perform a global reset. If your paired remote has been lost or damaged, you will need to use a new remote in IR mode to put the DVR in a pairing state, then pair the new remote to the DVR. For instructions, see To Pair a different remote to your DVR.
To perform a global reset:
Press and hold the TiVo + TV Power buttons until the activity indicator blinks red.
Press Thumbs Down three times, then press Enter.
The activity indicator will flash red three times if the global reset is successful.