Erratic MIDI

I have Cubase 9 Pro version 9.0.20. All of a sudden when I play in MIDI on an instrument track or in Kontakt with any instrument, I am getting notes that play back slightly late or early with no pattern - just erratic. I have tried 2 keyboards - same thing. I am on iMac Sierra OS. I have changed the buffer size to low, medium, high to no effect. It is not a quantizing issue. Does anyone have any ideas? Thanks MUCH! LR

First I come to think of is how sampler has preloaded samples or not.
There are various settings, some called smart that only load samples used and can take a playthrough before no delays.
Other setting preload every sample in memory and quick as it can be.
Other use kernel streaming which needs large enough preload buffer in sampler to work flawlessly.

Just a few ideas…