A B757'S INSTRUMENTS AND LIGHTS BEGAN FLASHING ON TKOF. TURNING OFF THE L GEN CTL UNIT RESTORED NORMAL ELECTRICAL. AN EMER WAS DECLARED WITH AN IMMEDIATE RETURN TO LAND.

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AS THE MAIN GEAR LEFT THE GROUND WE EXPERIENCED CONTINUAL FLUCTUATION OF LIGHTS AND FLIGHT INSTRUMENTS. WE FLEW OUT ON INSTRUMENTS WHEN THEY WORKED; STANDBY INSTRUMENTS WERE USED AS A BACKUP. WE DECLARED AN EMERGENCY AND RETURNED TO LAND AT ZZZ. SUBSEQUENT INFLIGHT TROUBLESHOOTING INDICATED THAT THE LEFT GENERATOR WAS RAPIDLY CYCLING ON AND OFF. WE TURNED IT OFF AND THE ELECTRICAL FLUCTUATIONS CEASED. WE STARTED THE APU TO GET A SECOND GENERATOR AND COMPLETED THE QRH PROCEDURE FOR LOSS OF A GENERATOR. DUE TO THE LIMITED FLIGHT INSTRUMENT INFORMATION AND THE LIGHTING; WE SLIGHTLY OVERSPED THE FLAPS DURING RETRACTION AND CLEAN UP. WE; ALSO; LANDED 11000 LBS OVERWEIGHT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE L GEN ITSELF WAS NOT THE PROBLEM BECAUSE THE L GEN WAS RAPIDLY CYCLING OFF CAUSING THE LIGHTS AND INSTRUMENTS TO FLASH. AFTER THE RPTR OPENED THE L GEN CTL RELAY THE SYSTEM SWITCHED TO THE R GEN AND THE SYSTEM STABILIZED. A REVIEW OF THE MAINTENANCE LOG REVEALED THAT AN IDG WIRING HARNESS REPLACEMENT CORRECTED THE PROBLEM.SUPPLEMENTAL INFO FROM ACN 808309: INITIATED A NORMAL DEPARTURE USING STANDARD PROCEDURES. IMMEDIATELY AFTER LIFTOFF ALL LIGHTS AND INSTRUMENTS BEGAN FLASHING ON AND OFF VIOLENTLY; INCLUDING ENGINE EICAS. WEATHER WAS TOO LOW TO AVOID IMC (700 SCATTERED; 12 OVERCAST AT DUSK). WE DECLARED AN EMERGENCY AND ADVISED THAT WE WOULD RETURN TO ZZZ. WE WERE CLEARED TO LAND ANY RUNWAY. MY PRIMARY FLIGHT DISPLAY WAS FLASHING ON AND OFF; SO I DEEMED IT UNRELIABLE; GLANCED AT THE CAPTAIN'S PFD TO SEE IT OFF; SO BEGAN FLYING BY STANDBY INSTRUMENTS. CAPTAIN BEGAN WORKING THE PROBLEM AND DELEGATED FULL AIRCRAFT CONTROL AND COMMUNICATION WITH ATC TO ME. I WAS SWITCHED TO DEPARTURE AND GIVEN A VECTOR FOR A RETURN TO ILS AND REQUESTED ABOUT A 20 MILE FINAL UNTIL FURTHER NOTICE. BECAUSE OF MY DEGRADED INSTRUMENTATION I BEGAN FLYING ATTITUDE/POWER SETTING COMBINATIONS WHICH CAUSED AT LEAST ONE FLAP OVERSPEED CONDITION. AT ABOUT 1500 FT AGL THE CAPTAIN RESTORED SERVICE BY DISCONNECTING THE LEFT GENERATOR CONTROL UNIT (USING SYSTEM KNOWLEDGE AND LUCK SINCE NO PROCEDURE EXISTS FOR THIS). THE APU WAS STARTED BUT THE APU GEN (TO MY KNOWLEDGE) DID NOT JOIN THE RIGHT GENERATOR. I FELT THAT THE CAPTAIN WAS GETTING CAUGHT UP AND COULD ADEQUATELY MONITOR MY APPROACH; SO I BEGAN SLOWING AND ADVISED APPROACH THAT WE WANTED A VECTOR TO FINAL. THEY COMPLIED; SO THE CAPTAIN REVIEWED THE APPROACH PLATE WITH ME AND WE ELECTED TO LAND FLAPS 25 DEGREES DUE TO AN OVERWEIGHT LANDING. WE DID NOT HAVE SUFFICIENT TIME TO REVIEW OVERWEIGHT LANDING PROCEDURES; AND WERE MORE CONCERNED ABOUT LANDING QUICKLY BECAUSE WE DID NOT KNOW IF OUR ELECTRICAL CONDITION WOULD DETERIORATE. WE LANDED OTHERWISE UNEVENTFULLY AND TAXIED TO A GATE.