R-129 Bose System - 94 SL

Several times a week, when either in CD or FM mode, sound will stop, and the letters "PHONE" are indicated on function screen. Sometimes after 10 min or so, it will return to normal function, sometimes the word "CODE" will appear. This requires entering the security code. When code is entered, sometimes it will function normally, sometimes the word "PHONE" appears again. The cycle will continue. Pot Luck whether you get function or "PHONE". Any ideas?
Regards,
RJCamp