Thanks for the suggestion, Audrey. When I turned my attention to the midi leads, I noticed that the behaviour immediately ceases when the midi leads are unplugged from the keyboard. I've given the leads a good blow, to no avail, but am yet to try a more refined method of cleaning them. Mind you, they are quite new and have been plugged in pretty much since I got them, so I would be surprised if they have had much of a chance to get dirty. I'll keep experimenting and report any progress.