First, check for updates. You really need to go to 8.1...even Microsoft doesn't really support Windows 8. Some people here highly recommend going for the optional updates as well.
The Octa-Capture had an update for Windows 8.1, maybe the Quad Capture did too.
Turn off "Low latency" mode in the Capture control panel.
Graphics cards can be a minefield of issues. Check for updates, and disable any HDMI driver it throws into Sound, Video, and Game Controllers.
Check for Kontakt updates.
Disable any drivers you don't need to use, like wi-fi cards.
If you're using a gamer mouse that can be a problem.
USB can be a problem, but that's a separate issue...
After doing all this, then if problems persist it's time to narrow in on specific symptoms.