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Jianbo,

You mention that you have enclosed your system with an incubator.  If the
drift you are experiencing is the sort of drift that Johannes is talking
about, enclosing your system will not help at all.  

First, you need to determine whether or not your drift is "real" (drift
caused by the automated stage moving itself because it is sticky) or not
(possibly due to thermal fluctuations causing expansion/contraction of
optical components).  If it is "real" drift, then your automated stage
should report the drift: just check the (x, y) position of your stage at t0
and at t1, before and after you notice a drift.  if there is a change in the
(x, y) position between t0 and t1, then the drift is probably due to a
sticky joystick or poorly calibrated stage.

If it is not "real", and enclosing your system with an incubator did not
work, then you may be out of luck.  You could try an objective warmer.  You
could also account for stage drift in your post-acquisition analysis or
get/write a software program to re-align your stage during acquisition each
time you go to take a new image (much like an autofocus program).

Dan