Problems with Bias and Flats?

[BlueAstra]

Can anyone give advice on this? I have an Esprit 150 and Starlight Xpress SXVF-H16 mono with filter wheel. In the past I got good results but I recently did a new set of calibration flats and bias. I now find that no matter what options (or program-I get the same result with DSS and PixInsight) I use, I seem to get bias vertical line artefacts in the final stacked image (which uses lights, flats and bias). I have never used darks with this camera. Can anyone indicate where I should start investigating to solve this issue. I have included a single sub, single bias, single flat and resulting stacked image for reference (all non-linear for visibility).

Master Bias.tif Master Flat.tif Stacked Image.tif sub C43 120.000secs Blue.tif