I think I suffered from the chromatic aberrations for a time. It looked as though I had a problem with the slit drifting across the star image, so sampling different parts of the image that had been colour dispersed by the Earth’s atmosphere.
Here is a plot that shows the result as the continuum varies, just looking at 2 points in a series taken 20 minutes apart. I’m glad to say I’ve not suffered from this problem recently.
Andy