AE unexpected preview pixelation

I've been happily putting several comps together. To date,  preview resolution has been great regardless of zoom settings or much of anything else.

Images, video text all look great when manually scrubbing the timeline. The same in pre-rendering. now, all of a sudden, I have a problem with extreme pixelization when previewing. Also the timeline indicator doesn't move with the playback.                

This followed a 16" MacbookPro "burp" which crashed the system and took a long time to reboot.

This unexpected, out of the blue crash has happened before  but never caused a problem with saved files.

Where do I start fixing this?