Forgot to answer this question. Some have reported that the screws that hold the boards down to the chassis have been loose. Certainly that COULD cause a problem since it is the 'job' of the screw to ensure the 'ground' portion of the board gets connected to chassis ground.
When I opened my GT30, all screws were secure with none loose. All were shiny and looked new. If by chance in some extreme environment, the screw should oxidize, then all s/b replaced. In my case, all screws were re-used and securely tightened. If you look carefully, you should see that the screw makes the contact with the solder-pad and it gets slightly crushed when tightened properly.