Possibly related to this: Hard hang post-mortem? - #8 by AndersG but I have not gotten around to editing the config files to remove the on-screen display
Anyway. This has happened a few times: You watch a recorded film from TVHeadend. After say 50 minutes it freezes. The Pi is not hung, you can exit with X and after a while the GUI comes back. You can naviigate, but it i e-x-t-r-e-m-e-l-y slow. Say a minute between keypresses. Saw nothing odd in top, but had to reboot as family wanted to watch the last bit of the film
Only other thing I noticed was that the SD disk led was flashing frantically.
Did a sudo reboot and watched the rest of the film