I think that it may be a failing driver. I pulled a known good driver out of my lathe and replaced the Z axis on the mill and the problem went away. So it appears I have a failing driver. I didn't realize that these drivers could sort of work. I thought a driver would either work or be dead but it seems to be 'failing'. For $30 it lasted quite a while and is a cheap fix.
After the driver swap I did install the UC-100 motion controller. I didn't notice any ground breaking performance increases but I would say the overall operation of the machine is 'smoother'. I'd been wanting to add one for a bit anyway to reduce potential windows issues.
I'll let it go for a week or two and will post back if the problem comes back. But certainly, if you've checked all of the connections, made sure it isn't a Windows problem, look at the driver itself. The running temperature is an interesting one. None of my drivers have reached 60 deg Celcius when running my programs. I just don't have enough sustained axis movement to heat them up. The KL-4030 are already at 'full current' and set to 3.0 amp. and the motors are rated at 3.0 amp also. not much I can do there. and everything did work fine for 18 months. to answer someone's question, yes, I use (3) KL-4030 drivers on the machine all running from a single 36Volt/9.7amp power supply.
Incidentally, I did move the 'bad' driver to the lathe, and it works fine. On these mills the Z axis clearly works the hardest so perhaps that is a contributing factor and maybe my KL-4030 and motor combo is a bit under-powered for the job. so it may be that I have been running it at it's limits for too long. with a lessened load on the lathe no issues. I'm replacing it anyway as it just isn't an expensive part and I enjoy making parts far more that working on the machines, lol.
thanks and I'll post back if it turns out the driver was not the culprit!