Hi Mick.

Sometimes the selected MIDI DRIVER can make a difference. To render from a VSTi, or DXi, you don't actually need any output drivers selected, so that's the first thing to try. If that doesn't work, rharv has said, previously, that selecting the MS Wavetable synth has been known fix it. Neither of us know why!

Does the problem still occur when rendering a track to an audio track, instead of a wav file? That might be another way to go.

ROG.