Tvheadend

I'm looking for some advice on debugging these issues. I'm pretty comfortable with the debugger & C and have some decent understanding of media streaming issues, just trying to figure out how to interpret this scenario.

Errors like this:

2014-02-09 16:00:40.980 TS: 677028/KING-HD: MPEG2VIDEO @ #49: Continuity counter error, 20 duplicate log lines suppressed

Specs:

First-gen raspberry pi, hardwired to 100mbit ethernet
Hauppage 950Q ATSC tuner
Linux up-to-date on the Pi
Same tuner works fine on a Windows PC at the same location (not a reception problem)
Target system in this case is a hardwired PC. No wifi.

Observations:

Around 10,000kb channels look like they're working fine
However, 1080i channels look to be in the 13000-15000 range, and I see frequent skips.
top consistently shows > 50% CPU idle
ifconfig shows no dropped packets or errors
overclocking the Pi does not appear to affect results
in tvheadend, status page seems pretty stable and does not show any bit errors for the channel
subscriptions, on the other hand, shows a large number of errors and lots of volatility - in particular, output often drops to zero.

Does this sort of thing indicate that the PI is having trouble reading video quickly enough from the USB controller, or difficulty pushing it quickly enough out the ethernet interface?

Is there a technique to narrow this down further to determine where the buffer starvation is occurring?