How to deal with shifting CCD bias (zero exposure) levels?
Looks like you've got some flaky electronics attached to your CCD if the bias level is changing by several units in consecutive exposures. However, all is not lost if your CCD has an overscan region.

This is normally several columns of "pixels" that appear appended to each row of your data, but which do not correspond to physical pixels on the CCD. They are actually dummy readouts to assess the actual bias level in each particular row.

The overscan region should be clearly visible in any real exposure of a light source, since the overscan region will be un-illuminated.

You can use the overscan in two ways. The conventional method is to overscan-correct your images before doing a bias subtraction. You should also do this to your bias frames before combining them into your "master bias" frame.

So, take your overscan region and either (i) calculate a median (or clipped mean) within it and subtract this value from every pixel in the image before use (including the bias frames); or (ii) fit something more complicated as a function of row number and then subtract this from your actual data on a row-by-row basis.

Method (ii) is required if you notice that there is any structure or drift/trend in the overscan values as a function of row number.

This link gives plenty of details, some pictures and a recipe.