Writing that last post felt like venting at the time, but it turns out to have value in laying out all the theories I have read and ideas about maybe what to swap out next.

After running a few hours in the garage with no errors, I mounted the scope and turned everything on just as if I was imaging. Mount errors almost immediately. The main difference? The 12v 10A supply I was using for the cameras and dew heater.

Went and found the 3A supply that came with the QHY183C and swapped it in. So far, no errors. Could this be it? Actually a combination of 'its'. Clearly the iOptron usb cable has issues, but there was also presumably RF noise being generated by the 10A power brick. We'll see. Cloud forecast is iffy for tonight, then nothing looking good for the next week.

Hi Ron,

Most FT232 USB-to-serial chips support baud rate 9600 only. Too fast may cause trouble to connection.

BR//Kenny

Hi Kenny,

Thanks for the suggestion. I did try various baud rate settings, including 9600, of course. Never seemed to make a difference. The common standard for recent iOptron mounts is 115200, so the current batch of FTDI interfaces can, presumably, handle the higher baud rate.

I am cautiously optimistic that the combination of a non-iOptron usb type-B cable and removing the 10A power brick has solved my problems, but I'm not declaring victory until I get clear weather and can run a few night's worth of successful imaging.

I had been using the same power brick for a year (mostly while using a CEM25P), but it is entirely possible that it has experienced some sort of change causing it to emit much more noise recently. Indeed, the small batch of subs I was able to collect on the nights with frequent repeated disconnects all look like this:
  

I am really hoping the color banding was caused by RF interference from the 10A power brick, and not by a failure of my camera.

I am looking at a similar problem, I believe it is the camera or the power supply and usually, it is a matter of grounding (earth loops) that cause this issue. I hope to look at it this weekend experimenting with my power supply.