Sorry to hear that the Clone stamp is causing flashes on the canvas. Could you please try deselecting "Use Graphics Processor" located in Photoshop's Preferences > Performance, then relaunch Photoshop and let us know if it helps?
I am having the same problem. Gave that suggestion a try, it did not solve the issue. I use the frequency separation process and it seems to mostly happen in that situation on the current layer being sampled, especially when zoomed in and tapping the brush repeatedly for fine texture correction. I checked out whether it was my wacom tablet or adobe updates, those efforts did not present any solutions. My OS is up to date. The screen flashing in my face makes it challenging to see what I'm doing, it's slowing down the workflow so if you have any other ideas it would be most appreciated! Thank you.