Video in viewer is soft when playing, fine when still....?

Been using the Viewer smaller while getting to know X, but just started working with a 100% playback window. Now I see that when I play any clip or timeline, the image degrades slightly - gets softer - when playing. Hit stop, it sharpens. I view the clip in Quicktime & it's fine. Start/stop, no shift in quality. Made sure that the timeline was same as clip (settings), no BG renders, etc. Tested with DVCProHD720/30p and AVCHD 1080 24p

Almost looks like it's adding fields. Aliasing.

Am I missing something?