Sudden skip back in timeshift buffer

[F1Fan]

Okay, this has now happened four times in about 3.5 months of usage, so it's not exactly common, but it's happened enough to lower the WAF.

I'll be watching 'Live' TV, timeshifted some period behind real life. I'm pressing no buttons and doing nothing but sitting on the couch watching. The screen will freeze for a second and then video will begin again, only I'll now be back very near the beginning of my timeshift buffer.

I should note that I run a 4500 bitrate at 352x480 since that seems to produce the best PQ on my current, old TV. I also have a 4088MB buffer, which translates to a 2 hr. buffer.

The one that happened last night went thusly: I was watching a movie that ran from 7-9PM. Since there weren't commercial breaks, I'd started out in real time but had paused a couple times for snacks. I think I was about 20 minutes behind real life, and about 20 minutes into the movie. In other words, it's currently 7:40, and my buffer covers from 5:40 to 7:40. My marker is at around 7:20. Suddenly, I'm watching whatever was on about 10 minutes into the buffer, or about 5:50.

This is the first hiccup of any kind I've had in the almost two weeks of using the new box as listed in my sig. I had this happen three times with an old Dell 800 PIII I used as a proof-of-concept and thought CPU overload might be an issue. I now run at 3-6% CPU for recording, and 4-13% for playback while recording. Any thoughts on what might cause this beyond random glitch are appreciated.