1) Have you tried another monitor?
Yes, just today. I have seen the same situation of my system: good photos appear good and changed photo appear changed. Therefore my monitor doesn't mislead
2) Does it only affect the images you have modified?
I don't know. I fine-tune all the photos I download
3) ... Does your image program actually save the modifications to the image
4) Have you tried saving the modified image to a different name?
Yes. Same results
4) If you answered YES to question (4): When the change occurred, did you then compare the original image with the modified image using a program, not your eyes, (eg. Beyond Compare, AntiDupl, XnView, etc)?
No. Spontaneous changes of photos are evident. No need to try other viewers
5) Have you tried saving the modified image to another format, (eg. PNG, TIFF), as well as the original format?
5) If you answered YES to question (5): When the change occurs, does the alternate format image exhibit the same change as the original modified format?
Yes, the same change
6) Have you tried copying an image you have modified to a flash drive, (then remove it from the computer)?
Yes, just today
6) If you answered YES to question (6): Did you then plug it into another computer and ensure that the image had been modified?
Yes, just today (see point 1)
I am not able to answer to other questions.