These are all good ideas and the BK and MC or anything else that uses a Sprecher Schuh type of relay, are always suspect. I would try swapping PMI cards with another car to see if I could move the problem. The modules do actually get old and the internal led diodes start to fail. This can be a bitch to find as they become very intermittent. Usually PMI 1,2 are your most critical but I have seen them throw an extra GL module on a PMI card further on down the line especially if one car in the group has a rear opening. One of the other critical things that fail is the main 5 volt power supply. I have seen the two remote sense adjustment start failing thus shutting the power supply down with the car in flight.