DVR intermittent failure to record

I have an intermittent problem where I record a show and it seems to record just fine, shows up on the list as recorded, but it has no content or sound or picture when I try to play it back. No error message; it obviously thinks it's playing the show, and it takes up disc space for the whole time period and the clock runs, but there's nothing there. It did it twice in a row recently on PBS Shetland episodes, but the next one recorded and played just fine, as well as no problems with other shows recorded during the same period of time. Rebooting doesn't seem relevant, since it works most of the time, so you can't tell if the reboot did anything, and the missing episodes never come back. Any ideas? It did it last year a few times too, and it's usually on PBS fist run shows.