Hello! There is suggestion that electrical interference might be causing problems with my older SBIG ST-402. I know little about such things.
Might a power line conditioner/noise suppressor help along with ferrite cores on power and USB lines to the camera? Best regards. Mike
Such things certainly won’t hurt it. 
Might help, might not depending on the actual source of the problem.
After lots of testing with the camera and looking at another ST-402, I’ve concluded that my ST-402 is fine. There is probably some electrical interference accounting for the negative pixels after calibration.
Since adjusting the offset/pedestal and/or changing images and calibration frames to 32-bit float seem to take care of the problem, I’ll leave everything as is.
Thank you all for looking at this through many posts and topics. Best regards. Mike