Had something similar with the Neximage recently also using K3CCD. Can't say that I've got a really good handle on the right settings to use so I'm not much use for an explanation. My approach is a bit haphazard, I just play around with the gain with white balance checked or outdoor, fluro etc and select freeze (if I remember to) when the colour looks about right. As with you though a last avi on the night of the 28th came up better and with a deeper hue than usual apparently of it's own accord- whether this was settings or seeing changes (thin cloud passing by might have played a part) I don't rightly know.
cheers,