Having performance issues with 3 BLI PA units running as MU. Every once in a while 15 to 20 minutes or maybe even an hour into running one of the locos makes a grinding noise in the mechanism as though one of them is locking up or hopping a tooth. They run very well individually and only act up when run as MU. Am I nuts or is there an issue? I did some research and did not find any bad reviews.