A B757-200 HAD PREVIOUS RPTS OF COLD COCKPIT AND THE TRIM AIR WAS DEFERRED PER THE MEL. INFLT CREW EXPERIENCED COLD COCKPIT. SWITCHED OFF L PACK AND COCKPIT TEMP BECAME NORMAL.

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I PICKED UP ACFT FOR A TRANSCONTINENTAL. INBOUND CREW ADVISED THAT THE TRIM AIR WAS PLACARDED INOP; AND THEY HAD TROUBLE CTLING THE TEMP IN ALL CABINS AND WERE PRETTY COLD UP FRONT. THEY ALSO ADVISED THAT THE PREVIOUS CREW RPTED WE FROZE OUR BUTTOCKS OFF. THE FIRST PLACARD WAS FOR THE FLT DECK TEMP BEING INOP. MAINT CHANGED THE PLACARD TO THE TRIM AIR AFTER A SECOND WRITE-UP. FOLLOWING TKOF; WE HAD THE SAME PROB -- EXCESSIVE COLD AIR IN THE FLT DECK AND FORWARD CABIN; THUS HAVING PROBS CTLING THE TEMP IN ANY CABIN. I FEEL THAT MAINT IS PRESSURED TO PLACARD ITEMS WITHOUT TROUBLESHOOTING JUST TO KEEP THE AIRLINE FLYING. I WAS PREPARING TO DIVERT THE ACFT FOR OPERATIONAL NECESSITY. FLYING FOR A 5+ HR FLT IN 65 DEG OR COLDER FLT DECK TEMP WOULD HAVE BEEN UNSAFE AND SUBJECT THE CREW TO HYPOTHERMIA. REVIEWING THE SYS AND FROM EXPERIENCE; I DECIDED TO TURN THE L PACK OFF; WHICH SUPPLIES 75% OF THE AIR TO THE COCKPIT. THIS SOLVED THE EXTREME TEMP PROB AND THE SYS THEN OPERATED NORMALLY ON ONE PACK. UPON ARR I WROTE UP THE L PACK. IT IS MY OPINION THAT A SIMPLE OPERATIONAL CHK OF THE AIR CONDITIONING SYS ON THE GND BY MAINT WOULD HAVE FOUND THE L PACK FAILED TO THE COLD SIDE. BUT DOING SO WOULD HAVE CAUSED THE FLT TO BE DELAYED; THUS SUBJECTING THE FLT CREW TO POSSIBLE COMPLICATIONS INFLT AND CAUSING AN AIR INTERRUPTION.