A B737-300'S LNAV WOULD NOT REMAIN ENGAGED AFTER IT APPARENTLY WAS PROPERLY ENGAGED. PREVIOUSLY; THE #1 ENGINE GENERATOR CONTROL UNIT CIRCUIT BREAKER POPPED AND AFTER A SECOND EVENT WAS MEL'ED. SEPARATELY; THE STANDBY HORIZON ALSO FAILED.

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WHILE CLBING OUT OF ZZZ WE WERE CLRED DIRECT ZZZ VOR. THE FO ACTIVATED ZZZ VOR AND WE ENGAGED AND CONFIRMED LNAV. SHORTLY AFTER (24500 FT) WE WERE QUERIED; 'ARE YOU GOING TO INTXN A?' WHICH WAS THE FIX IMMEDIATELY AFTER ZZZ VOR. WE CHKED LNAV AND IT HAD KICKED OFF. WE TRIED TO REENGAGE BUT WE RECEIVED 'NOT ON AN INTERCEPT HDG' MESSAGE. WE TOOK AN INITIAL VECTOR TO INTXN A; REENGAGED AND RECONFIRMED LNAV. AFTER APPROX 6 MINS (35000 FT) WE WERE CLRED DIRECT YYY VOR. WE CONFIRMED A SWING IN THE LNAV HDG AND LNAV ENGAGED. AFTER A FEW MINS WE WERE QUESTIONED AGAIN 'ARE YOU GOING TO YYY VOR?' WE CONFIRMED THAT WE WERE IN LNAV WITH THE CDI CTRED UP; YELLOW AND RED BUGS LINED UP. WE STATED WE WERE SHOWING OURSELVES DIRECT YYY AND QUERIED CTR WHAT THEY SHOWED. THEY GAVE US A HDG OF 080 DEGS AND SUGGESTED WE SHOW /W AS OUR NAV IDENTIFIER UNTIL WE COULD SOLVE THE LNAV ISSUE. AT THE TIME OF THE SECOND INCIDENT THE INDICATIONS WERE AS FOLLOWS: LNAV ENGAGED (BOTH THE BUTTON ON MCP AND FMA PANEL); CDI CTRED; YELLOW BUG AND ORANGE BUGS ALIGNED; APPROX 060 DEG COURSE DIRECTED AND DISPLAYED ON THE CDI; YYY VOR AT THE TOP OF OUR LEGS LIST; AND 081 DEG COURSE TO YYY VOR SHOWING IN THE 'BOX.' WE ACCEPTED THE HDG AND COMPLETED DISENGAGED LNAV; REACTIVATED YYY VOR AND REENGAGED LNAV AGAIN. THE LNAV APPEARED TO WORK CORRECTLY FOR THE REMAINDER OF THE FLT. BACKGROUND INFO: THE ACFT POPPED A GCU (G-3 CIRCUIT BREAKER) DURING PUSH/START AT PREVIOUS CITY WHICH MADE THE #1 GENERATOR INOP. WE RETURNED TO THE GATE AND THE GCU WAS REPLACED. ON THE NEXT LEG INTO ZZZ UPON SHUTDOWN OF THE #2 MOTOR; THE GCU CIRCUIT BREAKER POPPED AGAIN. THE GENERATOR WAS DISCONNECTED AND MEL'ED BY CONTRACT MAINT. UPON PUSHBACK FROM ZZZ WE HAD A STANDBY HORIZON OFF FLAG THAT COULD NOT BE REMEDIED. WE RETURNED TO THE GATE. AFTER DISCUSSING THE ISSUE; MAINT CTL SAID THEY DIRECTED THE CONTRACT MAINT TO RESET THE GCU CIRCUIT BREAKER WHICH SOLVED THE STANDBY HORIZON PWR ISSUE. WE PROCEEDED TO OUR DEST AND HAD THESE LNAV PROBS ON THE WAY. UPON LNDG THE GCU CIRCUIT BREAKER POPPED AGAIN. I BELIEVE THE LNAV PROBS ARE RELATED TO A LARGER PROB. THE LOGBOOK WAS ANNOTATED APPROPRIATELY INCLUDING ALL ELECTRICAL ISSUES AND THE LNAV ISSUE. MORE CLOSELY MONITORING OF NAV INSTS AND PERFORMANCE; ESPECIALLY IN LIGHT OF THE NUMEROUS ELECTRICAL AND EQUIP MALFUNCTIONS.