Well if the monitor from downstairs does the same thing, and the monitor from upstairs works downstairs, then I think it is some problem with the wiring. The electrician is probably correct about noise on that one circuit. Like I said before, check or have someone check to make sure there is not something else running on that same circuit. Another problem might be that your neutral wire(should be a white wire) on that circuit may be tied in to another circuit. Somebody may have used the same neutral wire on two different circuits. If this is the case, then there is the possibility of getting the noize from something plugged into the other circuit.
The circuits should have there own neutral that goes back to the breaker/fuse panel. What do you have, breakers or fuses? Is this an older house? I guess it is an older house because you do not have a ground wire?
I know it can get expensive, but your best bet is to run new wire from the panel to your outlets that have a computer plugged in.