This is not something easily reproducible but when it happens, PPro is literally useless.
I have looked at all the obvious fixes (buffer size is perfectly adequate at 2048 samples) but none seem to help.
2 HD streams of video, 2 stereo audio tracks - yet one day it is fine and the next? You cannot understand a word because of the terrible stuttering.
It's an i7 with plenty of RAM and no shortage of HDD - otherwise it would do this all the time instead of intermittently.
Any pointers will be gratefully accepted.
I have looked at all the optimization white papers & FAQ and none of it helps.
Only a complete reboot "might" cure it.
I've never had that happen myself. Some round here suggest the Asio4All driver to cure such ills.