I have had recently recovered from a major illness and returned to composing in Cubase. I am delayed from doing this by the problem described in the title of this post. For instance the attached file shows a simple sketch, with everything but the Electric Piano muted out. When I hit playback it sounds like clickety click. However when I export the track it sounds correctly. Note that drums (and other percussion instruments) sound fine. Also the instrument EP sounds OK when played from a midi keyboard.
Mainly looking to see if there were any Notes being doubled, perhaps as super short Notes. This can be hard to see in the Key Editor because one Note can be visually covered by another. But they’re easy to see in the List Editor. And that’s also not you problem.
Try sticking a MIDI Monitor on the Track’s MIDI Insert. This will show the MIDI Data being sent to the VSTi. Run it for a bit while the problem is occurring and screenshot the results. Its Window can be resized to show more results.
I found a solution. I have been using a basic Focusrite Audio interface because I was having occasional lock ups with my Fireface USB C. So I switched
back to the fireface and the problem went away. Maybe the Focusrite out into some odd state. Case closed.