Steve, that sounds to me like it could be a motor coupling problem. A little over a year ago I had a Z axis "drift" over long periods that turned out to be slippage in the solid coupling between the motor shaft and leadscrew end.
Description of the problem and my initial solution is at
http://www.cnczone.com/forums/showthread.php?t=43238 . I ended up retrofitting a nice Stafford 4-bolt split coupling that I bored to exact size and haven't had a problem since. I sometimes run 15+ hour contouring runs and the Z is still in registration at the end.
Best regards,
Randy