I was having issues with some/most videos stuttering, but some working properly, and this is after a long period of near perfection using mostly the same library. Problems occurred using the LG plex client, fire HD plex client, xbox one plex client, and clients on both iphone and ipad (ios).
misdirection: This lead me to believe there may be filesystem problems, a failing drive, or a failing RAID. After messing around with the qnap (TS-253A) filesystem utilities - garbage (it can't even unmount the filesystem to do a check, long story), decided was nearly impossible to recover from, so freshly reinstalled and formatted everything, built RAID from scratch, decided to download stuff again. SAME ISSUE STILL.
TLDR: concluded plex was the culprit of the sudden stuttering on a wide variety of types of media, downgraded plex media server from 1.0.0.2261 to 0.9.16.6 got it working perfectly again.