CORP LTT INVOLVED IN TRACK DEVIAITON DUE TO INS NAVIGATIONAL PROBLEM WITH PMD VORTAC WHILE ON VNY FIVE SID.

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DURING CLIMB OUT ON THE VAN NUYS 5 DEP; PALMDALE TRANSITION. ANOTHER ACFT WE THOUGHT TO BE AHEAD OF US; ASKED APCH FOR 21000'. APCH SAYS; 'I JUST DID.' THE OTHER ACFT REPLIES; 'OH; I'M SORRY.' APCH SAYS IN A PUNISHING TONE 'I'LL GIVE IT TO YOU AGAIN.' WHICH HE DID; AND THEN IN THE SAME VOICE SAYS; 'GIVE ME A READ BACK.' OTHER ACFT COMPLIED. APCH THEN STATED 'IN THE FUTURE; IF YOU DON'T INTEND TO GO OVER THE VOR (PALMDALE 114.5) ADVISE APCH.' OTHER ACFT REPLIES; 'WE'RE DOING THE BEST WE CAN BECAUSE OF THE VOR SIGNAL.' HE SAYS; 'WE HAVE COMPLAINED ABOUT THIS FOR OVER A WEEK.' SOMEWHERE ABOUT HERE IS WHEN I STATED THAT WE ALSO WERE NOT RECEIVING A GOOD SIGNAL BELOW 13000'. WE WERE OUT OF 15000'. OUR INS DROPPED OFF LINE DURING LINE-UP ON RWY. APCH IMMEDIATELY JUMPED ON OUR CASE AND GAVE US A HEADING FOR VECTORS; THEN IN THE SAME TONE ASKED US ABOUT WHAT NAV SYS WE WERE USING. I STATED THAT WE WERE TRYING TO COUPLE THE INS AND USE THE VOR AS WELL. THERE WAS SOME OTHER BRIEF CONVERSATION THAT TRAILED OFF AND SEEMED INSIGNIFICANT. APCH THEN GAVE THE OTHER ACFT A COLLECT PHONE NUMBER AND TOLD HIM TO CALL WHEN HE GOT ON THE GND. WE WERE GIVEN ANOTHER FREQ AND THOUGHT NO MORE OF IT UNTIL 10 OR 15 MIN INTO THE FLT; LA CENTER GAVE US A PHONE NUMBER; SOMETHING ABOUT PLT DEVIATION.