Sorry to hear you are still having problems. Sporadic problems are the worst.
Perhaps someone else here who is better at reading logs than I, will pipe up with some suggestions.
If it were me at this point (given that I have a lot of mistrust and bad experience with usb, and given that you have potentially eliminated the PC as root cause), I would:
1) Try swapping out the QSI factory supplied cable with a different cable
2) Having a closer look at your other usb devices and hubs/cables - try swapping. Also need to consider there could be multiple issues at play. Are you using a usb hub? If so what make/model? Usb 2 or 3 cables?
3) When you did the "isolated" QSI testing (ie. no other devices connected) how long were the subs? If short, perhaps try longer subs to see if it makes a difference (help narrow down the problem at least)
4) Does your QSI camera have any logging?
5) I presume you have checked for newer version of drivers (camera, mount etc.) and applications (ie. ASCOM, PHD etc.)?
Maybe someone else here with similar equipment/combination can advise if they have ever had similar issues before?
Just throwing out some ideas ... wish I could suggest something more concrete.