If the chamber bath height is not stable it is usually due to the formation of a meniscus at the aspiration tube.  This can lead to an uneven aspiration rate.

There are two simple ways to improve bath height stability.
1.  Use a strong suction on the aspiration end.
2.  Put a small piece of Kimwipe in the suction well and aspirate off of the Kimwipe.

Both techniques prevent meniscus formation at the aspiration tube.