What was found
ELEC GEN SYS L EICAS STATUS IDG HAS DISCONNECTED OR A PROTECTIVE TRIP OF THE GENERATOR CONTR OL BREAKER HAS OCCURRED OR A PROTECTIVE TRIP OF THE BTB HAS OCCU RRED. RESET PER ECL RESET SUCCESSFUL DEBRIEFED FLT CREW LEFT IDG DISCONNECTED 2 HRS INTO FLIGHT ACCOMPLISHED RESET LASTED 20 MINUTES DICONNECTED AGAIN AIRCRAFT RETURNED TO EWR DISAPPEARED. PERFORMED FIM TASK 24-21 TASK 817 IDG OPEN CIRCUIT PROBLEM. ALL WIRING CHECKS ARE GOOD. REMOVED AND REPLACED LEFT GCU PER AMM TASK 24-22-02-400-801.NEED ENGINE RUN DUE TO HISTORY. ELEC GEN SYS L MESSAGE WAS THERE BEFORE WE REPLACE THE LEFT GCU BUTUNIT, GENERATOR OFF 25-2422-9-0002 0617 ON 25-2422-9-0002 0719 PERFORMED LEFT GCU SOFTWARE LOAD IAW AMM 24-22-02-400-802 AND LEFT GCU INSTALLATION TEST IAW AMM 24-22-02-710-802. TEST PASSED. LEFT ENGINE RUN WAS PERFORMED AND NO FAULTS NOTED.