1) If the Z DRO reads the correct number, than my guess is noise on the encoder wires. That should be the only way a servo can lose position. I'd double check everything for mechanical issues. Put marks on pulleys and shafts to verify they are not moving. I can't tell you how many times people think they know for sure a problem is not mechanical, and it turns out to be mechanical.
2) Not sure on that one. If you can find a program where it's repeatably, try running Task Manager and monitor CPU usage when it occurs. Could possibly be something running in the background.
Does the Feedrate DRO actually show it going to 80?