I know you prefaced this by acknowledging that what you tried is "a little nuts," but it doesn't sound like a good idea. Even if these problems are caused by some kind of interference--
which Oppo has ruled out--aluminum foil is not a good EMI/RFI shield. It's garbage. In fact, it could act as an antenna. Instead, you'd need nickel, copper, or silver mesh. Also, opening up your device and putting stuff in it will void your warranty. If there is a fix that involves sending your unit back to Oppo, you may be out of luck. Then again, Oppo is so good to its customers that maybe they would accept it back for repair. Not sure, though--putting stuff inside falls into the "you should know better" realm!