Rutex Z axis problem--more info

Just to recap the problem I'm having. When ever I make a Z axis
movement it travels farther than it should by about .015". It does
it in the Z+ and Z- directions. It's not always .015" sometimes its
less. Also it's intermittent. It seems to be good in the morning
when I turn it on but over time it starts to happen and when it does
it's intermittent. A move as small as .001 will cause it to happen.
It's maintaining it's accuracy, .001" gives me .001" of movement on
a dial indicator. I can cut for hours and not loose any steps. If I
power up the servo amps alone there's no problem but when I turn on
the drives the Z creeps down .015" then slowly back up.
I don't think it's a tuning problem. I tried switching the drives
and it made no difference. I then tried switching the servo amp
itself and I still had the problem.
I'm wondering if it may be an encoder issue.

Any thoughts on this?

Thanks
Derek