You probably have seen my posts on this topic in other forums. It had been an extremely frustrating problem for me - nothing seemed to help. I have now taken these steps which have eliminated the problem for me over the past 30+ hours of imaging:
1. I swapped out ZWO's USB3 cable for a new one.
2. I always run the camera at USB3 with a USB setting of 40.
3. I always plug the camera directly into the back USB3 port of my PC, not the front USB3 ports.
I need to do all 3 above or I have the downloading problem.
My take on this is that the camera USB3 driver is not very stable. It troubles me that I have a new, state of the art PC and still I cannot run the camera with the front side USB3 ports for some reason. My guess is that your problems are USB3 port related.
Good luck - I know how frustrating it is.