I had a thought reading the reply below.
Is "anything" grounded, and are "all" things in use at the time grounded to the same place?
NOT just the "third wire" AC grounds, but ground screws (if any) on the "other stuff".
The original poster might want to look up what a ground loop is and how to repair one.
OR make some electrical measurements from one piece of equipment to another.
Sometimes you have "a leak" you can't find, and that may or may not point to something
"breaking down". 
It might be good to do all the above with (protected by) a good, functioning GFI,
just to be "safe" from something totally un-expected.
(Literally, it would not be "totally unexpected" since you are looking for "leaks"...)

This is where I "bow out" since we are getting away from computer issues and into
a potentially dangerous area...

                                                 Rick Glazier

From: "David Farrington" 
>I had an issue that sounded just like this one and what I discovered was 
> I had a ground loop problem between the equipment. (clipped.)

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