Ground Plane Issue
There was this circuit board that incorporated a gate array. The device came in a 191 pin package and it behaved a little cranky now and then. After some significant troubleshooting efforts, we concluded that the problem we were having came from the following condition.
Several signal carrying lines had been routed right under the gate array's package and although there was a metal package lid, that lid was electrically floating. We had EMI getting transferred from those signal lines to the chip inside the package.
It wasn't a difficult problem to solve. We changed the board layout from what you see above to what you see below.
This rerouting of the signal lines to the other side of the ground plane quieted things down just fine.