Hi Paul,
I’ve not seen this problem with any of the atik cameras I have used but I think rotating the camera 180 deg should quickly show if that is the problem. A similar effect can produced if the CCD is not mounted precisely square (ISTR some cameras having an adjustment) or the camera adapter out of square, though that would not normally be a progressive deterioration and if this is the case it would still be possible to focus well at any particular wavelength, just not everywhere at the same time.
Cheers
Robin