I'm having the random horizontal banding issue in both dark and light frames.
Also the dark frame seems divided into two zones by this band, one with narrow black stripes running down to the bottom and one (the upper) with no such stripes.
I shot 16 600s dark frames today and the banding is in all of them but drifts up and down and so remains in my master dark.
I also discoverd the banding in 600s light frames shot at a dark sky 15 days ago.
I use QHY9 v3.3 drivers (the latest that are on the QHYCCD front web page) and Tom's ASCOM 188.8.131.52 driver (also found in the link in the front QHYCCD web page).
I use WindowsXP, MaximDL, Gain=0, Offset=115, Overscan correction, all other settings at default.
I have not tweaked the fine tuning at all.
I tried to find the QHY9FW.sys file on my computer and replace it by the one you gave in a previous post but there's no such file (does not exist in v3.3 drivers)?
It is not a power supply issue because I shot the darks at home with a 12v power supply unit and the lights with my car's battery.
What is going on?