m_vogel These usually occur at the beginning of the night, but not always.
Patterns that appear at only a part of the frame, and especially when they go away with time, is often caused by frost on the sensor. If the bumps slowly become smaller with each frame, until they finally clear away completely, it is almost certainly cause by frost on your sensor.
The usual cure is to not cool the sensor to the target temperature that is below 0ºC right away. Do it slowly in multiple steps, or at least two steps -- first cool to a couple of degrees above freezing, let it sit there for 5 minutes or so, and only then take it down to the target temperature.
The behavior is getting worse and worse lately.
Are your nights becoming more and more humid lately?
If the problem keep getting worse without weather coming more humid, you may need to re-bake the desiccant tablets that are in the camera chamber. The less moisture in the chamber, the less chance to form frost on the sensor.
Check the ZWO documentation on how to do recharge desiccant tablets. If you are in friendly terms with your local dealer, they may be willing to do it for you.
Chen
