I guess you can mount upside down, that is why you can rotate the picture I assume, It is worth a try right side up and see if it keeps going off line. I looked on the forum and it said code 90 is the cam is not connected to the network which you already know, Strange it would keep doing it at different times. Is the power stable and the connection from the power source tight. Some times it is hard to get the USB cable male end inside that little boot cover on the end of the camera cable without breaking anything. Since you have not mounted it again yet I would just plug it in close to wireless router and let it run for a while and see if it goes off line. I am sure someone smarter that me that has had the same problem will be along and give better answers. How far is the cam from the power source?
Here is an old thread, and long read on an issue like yours