B737-300 PLT HAS FLASHING CTL DISPLAY UNITS AND RADAR SOURCE WARNING MESSAGES. PAX WERE FOUND TO BE USING WALKIE-TALKIE EQUIP IN THE CABIN.

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AT CRUISE ALT OF FL310 WITH RADAR 'ON;' ALL 4 EFIS DISPLAY UNITS STARTED FLASHING AND BLANKING OUT; ACCOMPANIED WITH A YELLOW 'WX RADAR SOURCE' WARNING FLAG ON BOTH EHSI DISPLAY UNITS. I TURNED BOTH RADARS 'OFF.' ALL DISPLAY UNITS STOPPED FLASHING AND BLANKING OUT. I TURNED FO'S RADAR BACK 'ON;' FLASHING SEEMED TO HAVE STOPPED. FLT ATTENDANT CAME TO THE COCKPIT AND ADVISED THAT 2 PAX HAD WALKIE-TALKIE TYPE HEADSETS (MODEL DIGI-TECH DSI) ON AND THEY WERE TALKING TO EACH OTHER. THEY WERE SITTING IN DIFFERENT PARTS OF THE CABIN. I ADVISED THE FLT ATTENDANT TO HAVE THEM TURN 'OFF' AND DISCONTINUE USING THE HEADSETS. I TURNED BOTH RADARS BACK 'ON;' RADARS AND ALL 4 EFIS DISPLAY UNITS WORKED NORMAL FOR THE REMAINDER OF THE FLT. I STRONGLY SUSPECT THAT THE RADAR WAS RECEIVING ELECTRICAL INTERFERENCE FROM THE PAX'S WALKIE-TALKIES. FLCS NEED TO CONTINUOUSLY BE AWARE AND MONITOR CABIN AND PAX FOR USE OF UNAUTH ELECTRONIC DEVICES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT THE FIRST INDICATION WAS THE RADAR SOURCE WARNING MESSAGE FOLLOWED BY INTERMITTENTLY FLASHING OF ALL THE CDU'S. TURNING OFF THE RADAR UNITS INITIALLY RESTORED OP TO NORMAL. ONE RADAR UNIT WAS TRIED AND SYS WERE INTERMITTENTLY FLASHING AGAIN. AFTER LEARNING THAT THE WALKIE-TALKIES WERE IN USE AND SUBSEQUENTLY REMOVED FROM USE THERE WAS NO FURTHER PROB RPTED. THIS WAS THE SECOND LEG THAT THE CREW FLEW THIS ACFT. THEY RPT NO PREVIOUS PROBS. NO MAINT LOG ITEM WAS FILED SINCE THE RPTR FELT SURE THAT THE PED WAS THE CAUSE OF THE PROB.