Auti focus curve like a W

[labtech1122]

Hi, I've had this weird thing happen to my auto focus runs this evening. When it runs it goes through the curve then starts going back up again as if it knew the correct focus then goes down again. So odd. I tried restarting asiair, resetting the focus tube position, checked all imaging train and everything looks OK. It's like on the point of actual focus it thinks the stars are bigger.

The only thing I have changed since it worked OK last time was I swapped a CLS filter for a antlia quadband filter (the antlia is the filter fitted in the images)

[labtech1122]

I did notice with a preview shot there are some halos around the stars (which I am under the impression is normal) the only explanation I can think of is when the stars are in perfect focus the halo is throwing off the auto focus software?!?