Problem- R2E3 Intermittent axis limit

[Jadaan27]

I am getting an "Axis Limit" fault when executing a rapid move with the Z-axis. It seems to occur after the Z-axis makes a rapid move of anything greater than a half inch or so in either plus and minus directions.
The fault is random, sometimes I run the machine for an hour without a fault, sometimes it will fault 6 or 8 times an hour. But it is slowly becoming more common. So whatever the problem is, it's getting worse.
When the fault occurs I re-enable the axis drives and re-home the machine and everything runs as it should until the fault occurs again.
I checked the servo controllers and didn't see any alarm on those.
The green lights on the FMDC board are blinking as they should.
I checked wire connections on the Z-axis home switches, all those looked fine.
Motor cables are tight.
Could this be encoder or motor problem? Anyone have any ideas?
Thanks in advance.