B737-300 FLT CREW REPORTS ANOMALOUS COMMANDS FROM FLT DIRECTOR IN LNAV MODE. SUSPECT ELECTRONIC DEVICE INTERFERENCE.

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I WAS PLT MONITORING; DEP RWY 7L AT A8 INTXN; COWBY2 RNAV SID. ALL PROCS FOR RNAV DEP PER FOM GUIDANCE COMPLETED PROPERLY AND VERIFIED. AFTER DEP AT APPROX 2600 FT MSL; LNAV SELECTED ON FLT DIRECTOR AND IT BEGAN TO COMMAND A SLIGHT R TURN. PF BEGAN TO FOLLOW GUIDANCE AND THEN THE FLT DIRECTOR CONTINUED TO COMMAND R TURN. LEGS PAGE SHOWED WASTE ON L1 AND CORRECT TRACK. I STATED THERE WAS AN ISSUE AND WE SHOULD CORRECT BACK TO A 075 DEG HDG. AT THAT TIME DEP STATED WE WERE S OF COURSE; HOWEVER; THE CDI SHOWED TRACKING ON COURSE THROUGH THE ENTIRE SEQUENCE. ATC GAVE US DIRECT COWBY WAYPOINT; AND THE WAYPOINT WAS SELECTED AND EXECUTED AND COUPLED WITH FLT DIRECTOR AND AUTOPLT. ACFT FLEW TO WAYPOINT; AND THEN APPROX 60 SECONDS LATER BEGAN AN UNCOMMANDED L TURN AND CONTINUED UNTIL LNAV WAS DISENGAGED BY THE PF. I REQUESTED FROM ATC WE HAVE A HDG AND TO DISCONTINUE THE RNAV DEP DUE TO UNKNOWN ISSUES WITH THE FLT DIRECTOR. NO FURTHER INCIDENTS OCCURRED DURING FLT. POSSIBLY CHANGE MASTER FLT DIRECTOR; HOWEVER; DUE TO THE CLOSENESS TO THE COURSE AND TIME RESTRAINTS; THIS WAS UNLIKELY. SUPPLEMENTAL INFO FROM ACN 779311: THE FMC WAS PREFLTED; VERIFIED AND RE-VERIFIED AS CORRECT BY BOTH THE PF AND PNF. THIS WAS A VERY STRANGE EVENT AND WE CAN ONLY SPECULATE THAT THERE WAS SOME SORT OF INTERFERENCE (CAUSED BY CELL PHONES/ELECTRONIC DEVICES; ETC). DISPATCH; MAINT; AND ATC REPRESENTATIVE NOTIFIED OF EVENT. STRESS COMPLIANCE WITH USE OF PORTABLE ELECTRONIC DEVICE (PED) REGS.