In the recent past I have speculated that a range of severe performance degradation, instability and glitch issues when using various forms of usb midi with he Quantum is caused by some kind of internal midi feedback loop.
Maybe this does not cover every issue people have had, but I think it is a worthwhile thing to try first, and my setup got a lot more stable when I did this myself (in my case with usb midi controllers rather than a computer connection but the same principal applies)...
Go into midi input and output settings on Quantum and set all the midi interfaces/ports you aren't using to off. If there are any left that you want to use for both input and output, set the midi in channels for that device to be different to the midi out channels.