It should be mentioned that the user (Tony) is using an external MIDI synth, not the usual soft synth Dxi setup. We're not sure why this issue is happening on his machine, and we can't duplicate it, either with soft synths or an external MIDI setup.
So we plan on sending Tony a test version to see if it fixes it, and at least provides us some diagnostic info.

Tony has a fast machine (Geekbenchy=6600). We've tested on fast and slow machines (Geekbench=1,000) and also can't duplicate any problem.

There are issues with some video cards and video acceleration, where they take over the whole system during redraws, and if that is the case disabling video acceleration makes the issue go away. info here: http://www.wikihow.com/Turn-off-Hardware-Acceleration

Also, if Tony is running from a USB drive, he might try this:
"was getting a major stuttering on my laptop when I used the external USB drive, what I ended up doing that solved it for me was going into Device Manager, selecting Disk drives > Properties > Policies > and changing the Removal Policy from Quick Removal to Better Performance the stuttering I was experiencing is now completely gone." (tech info: that restores write caching to the drive).

Have Fun!
Peter Gannon
PG Music Inc.