It sounds like a loose connection or something.
Just to check, have you tried hooking the LD player up to a TV to make sure it's playing alright? Even if it isn't you should at least get the "game" overlay as that will work even with no LD player present.
Those old LD players mostly have BNC connections, but you can (or could) get an adaptor at Radio Shack for a few bucks to hook it up to a standard RCA jack.
Does the monitor have power?