Poke, sometimes this is caused by a neighbor that also has an E* receiver. The 811s have a UHF remote control that can transmit quite a distance. Try changing the remote address to a different number to see if this helps.
In quotes the remote has a 150 ft. range so somebody close but not necessarily next door can change your 811 and you can change theirs. The remote is sent set for address #1 and most installers don't change it so there are thousand set for your address.
