I agree with tweeterguy that it's not likely the plasma's the cause. Haven't had to remove an antenna from an MRF-260, but it looks like in your case (with the number of remotes and base stations being used) that none of them ever get very far away from a base. Removing the antenna to restrict range might at least reduce, if not solve the problem. Also, tweeterguy's suggestion about making sure like devices are on different line numbers couldn't hurt.

Mike