A B767-300 CAPT RPTS A CONTRACT MAINT ORGANIZATION'S FAILURE TO CORRECTLY MODIFY THE TRAILING EDGE FLAPS. MANY RPTS OF 'FLAP DISAGREE' CAUTION.

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OUTSOURCED MAINT APPEARS TO HAVE CAUSED A LOW LEVEL ABORTED LNDG AND MISSED APCH FOR US TONIGHT AT ZZZ DUE TO THEIR MIS-ACCOMPLISHMENT OF ENGINEERING CHANGE ORDERS X AND Y WHICH WERE MAJOR REPAIRS DONE TO THE FLAP SYS. IN CRUISE WE GOT A SPORADIC 'FLAP/SLAT ELEC' STATUS MESSAGE WHICH IS A CHRONIC WRITE-UP. ON FINAL TO RWY 28R AT ZZZ WE GOT A TRAILING EDGE FLAP DISAGREE CAUTION WHEN GOING FROM FLAPS 20 DEGS TO 25 DEGS. AFTER GOING AROUND; AND ACCOMPLISHING THE APPROPRIATE FLT MANUAL PROC; WE LANDED FLAPS 20 DEGS WITH NO PROB. EVER SINCE THE AIRPLANE CAME OUT OF ZZZ1 IN MID-DECEMBER LAST YEAR IT HAS HAD DAILY FLAP/SLAT ELEC STATUS MESSAGES AND AT LEAST ONE TRAILING EDGE FLAP DISAGREE BEFORE WE FLEW IT. OUR MAINT DEPT REMAINS STYMIED BY THIS PROB. ANY REVIEW OF CARRIER MAINT HISTORY ON THE AIRPLANE WILL SHOW APPROX '42' THAT'S RIGHT; 42 LOG ITEMS FOR THIS PROB IN LESS THAN 1 MONTH! I DON'T THINK THIS IS A SIT THAT FLT CREWS SHOULD HAVE TO ENCOUNTER; DOING LOW LEVEL GAR'S AND RUNNING IRREGULAR CHKLISTS JUST BECAUSE COMPANY IS TRYING TO SAVE A FEW PENNIES ON OUTSOURCED MAINT. ALTHOUGH WE HAD A SUCCESSFUL OUTCOME; SAFETY WAS NEGATIVELY AFFECTED TONIGHT.