I had this problem before.same situation..went through all the motions with sending one car up to the top floor shutting one down,followed by resetting the first car waiting several minutes to turn the next car on..changed out both hall button interface boards..logic power supply in suspected car..but nothing corrected the problem until we replaced the 1k pull down resister in the lead car to a 750ohm.this prevented the 24vt logic for the cpu to operate properly.under extreme conditions, car calls,etc.kept the processor from clipping..