Advanced (x2) deinterlace is very heavy on sdram bandwidth.
Some PVR backends have little tolerance to being delayed and may drop packets when sdram bandwidth is heavy.
As you have found, increasing the arm's uncached (sdram) priority can help.
Have a read here: showthread.php?tid=269814&pid=2373475#pid2373475
The v3d_limited is another dial you can adjust which tweaks how advanced deinterlace uses sdram.
You can also try overclocking sdram which will help. Overclocking core and v3d may also help.
Options are:
Run PVR backend on a separate Pi from the one you watch.
Switch to Bob (x2) deinterlace
Switch to Advanced (x1) deinterlace
Play with some of the overclock and sdram controlling settings. Perhaps we'll find a better set of defaults.
Note: Jarvis defaulted to Bob deinterlace for HD, so switching back to Bob on Krypton should behave much like Jarvis did.