I have two AM21s attached to two receivers. I have a large antenna in my attic which feeds these. The coax comes into an equipment room and then splits and goes another 6 feet to each am21. I get 95-100% on every channel except one. In my case that channel is NBC. Here is the strange part. At times I will have a signal of 90% and above then out of the blue it will drop to 7% or even zero%. I have learned if I disconnect the coax where it comes into closet and then reconnect I will once again get a decent signal. This strange behavior is only on this channel. All others remain fine even when this one drops out. Any ideas?