Thanks for the replies.
I am seeing these issues in Cubase 7.07 with Toontrack drums and Musiclab RealStrat. There is no indication that anyone else has the same problem (exactly). Toontrack didn’t used to do this.
The problem is between Cubase and (Some!) VST Instruments.
I can’t see how latency could be an issue. No other VST Instruments misbehave and VST occupancy never exceeds 30%.
Somehow the flow of MIDI (buffering?) between Cubase and these plugins is scrambled briefly, then recovers. (Buffer not initialized?)
Is there anything in my MIDI configuration (Windows and/or Cubase) that might be causing this?