My guess would be the OTA rcvr circuitry and interface with HDD.
Mine will give me signal loss on OTA while getting a perfect signal with TV tuner. Recently it has started recording OTA timers at odd hours and for extended periods. For example, Monday, 2/10/14,. it started recording the timer set for local news from 6:00 - 6:30 PM on Sunday. Info reported original air date 2/16/14. When I turned on set to watch local news Monday evening got the signal loss error, hit DVR button twice and it showed the 'Sunday' recording still underway with something like 16 hours of recording. Selected view and got a live picture, stopped the recording and deleted then watched as normal. It has done this numerous times in the past month, wiping out what ever is on drive.
Had Mary with DIRT schedule a Hopper. When tech arrived he told me I would not be able to diplex in OTA. Only one cable in to house in conduit). Canceled the Hopper and had him order a replacement 622. It arrived DOA Friday. Now have a tech scheduled for Monday. We'll see how it goes.
Guess come spring I'll just have to pull another cable to house and give Hopper a try. I have a feeling these 622's are all suffering from old age.