If so, there's a "get cover art" Task under Configure Auto Import which may be automatically grabbing art for you. It would be bad if that over-wrote existing art, but that's all I can think of at the moment.
This is what I was thinking, but I just ran a test against one episode of a series, and MC didn't overwrite the Cover Art I had assigned from a random image. MC correctly saved the Cover Art file name (and location when not next to the video file), and retained that information through an Auto-Import process.
Note:
With Series and Season Cover Art, MC would have asked you if you wanted it to copy the Cover Art image to the default Cover Art directory, as per the Options settings, within an appropriate sub-directory. With Episode Cover Art if you answer yes to that question, the Episode Cover Art gets saved to the folder next to the video file. The file is also renamed to match the video file name, or Series or Season as appropriate. You should say yes to that question, usually, unless you really want to keep the Cover art images in another location.
If you answered No to the question about copying the Cover Art, MC will record both the directory name and file name of the Cover Art in the [image] tag.
So, did you say Yes or No at step 5?
Which Cover Art are we talking about here, Series, Season, or Episode?
What do you mean in step 7? After adding a random image to my Episode, I was able to use the "Cover Art\Remove Cover Art" function to remove the link to the image file I selected. MC then displayed what looked like a frame grab from the video file. I then added back the correct image using the "Add from file" function. What didn't work for you?
Also, when you say the Cover Art has changed into something else, do the new images match the images on TheTVDB, or do the image look like frame grabs from the video?
Do you have "Get Movie & TV Info..." turned on for the directory you are watching in Auto-Import?
The only thing I haven't tested is if Auto-Import is automatically run, does it overwrite any of the three types of Cover Art; Series, Season, and Episode. I don't expect it would though.