Some notes sustain, others don't trigger

I'm getting weird MIDI behaviour running Virus TI in Logic. It's hooked up via USB. Some notes hold, as if the sustain pedal is on, and other notes don't trigger at all. It's a different region or set of notes every time and I have checked the polarity of the sustain pedal and looked down to see if my foot is stuck to the thing, but no...

Anyone any ideas?
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Intel Mac 8 Core 3.2GHz / 10 Gb RAM / OS10.5.8 / Logic 8 / Apogee Rosetta 800 / API Summing / Crane Song STC8 and Ibis / Neve pres

I'm getting it with VSL now, so it can't be a Virus problem - must be Logic but haven't a clue as to why.