Jester,

I am curious if you would get the same results just grounding the 990 to the common ground plug on the power distribution center (PDC) and not grounding the 7700 to the PDC. My guess is "yes" as the 7700 has three prongs that connect to the power cable while the 990 only has two. I think you should be able to remove the one from the 7700 and elicit the same results.