So my current working hypothesis was that the problem was with nouveau driver - at some point Ubuntu for some reason decided that having NVidia drivers is a bad idea and reverted to nouveau. Upon taking considerable time to disable and blacklist it, things returned to normal. This is working fine now, will be continuing to monitor
It was actually message from @wendell that prompted me to ssh into the box and look at dmesg - where I spotted nouveau messages all over the place, so kudos for that!