DC10 FLC HAS ALTDEV DURING ARR INTO LAX.

Narrative

WE WERE EXECUTING THE PDZ 4 ARR INTO LAX. WE WERE ON THE SEGMENT BTWN TEJAY AND ARNES. WE HAD BEEN USING THE INS FOR NAV; BUT HAD JUST CHANGED TO 'HDG SELECT' TO ANTICIPATE THE INTERCEPT OF THE LOC AFTER ARNES. WE HAD STEPPED DOWN FROM 12000 FT TO 10000 FT FOR ARNES. I WAS MONITORING THE LOC ON MY SIDE (CAPT) AND THE FO WAS MONITORING THE PDZ VOR 277 DEG RADIAL RAW DATA; BUT WE WERE FLYING ON THE INS. THE INS SWITCHED FROM 'TO' ARNES; TO 'TO' SUZZI. WE SET IN THE NEXT LOWER ALT THINKING WE WERE PAST ARNES AND STARTED DOWN; OUT OF 10000 FT. WE STARTED TO TURN IN TO THE LOC WHEN THE CTLR TOLD US TO CHK OUR ALT AND GO NO LOWER THAN 8000 FT AT FUELR; IT WAS THEN THAT WE REALIZED WE WERE STILL 2 MI SHORT OF ARNES FROM THE DME AND AT 9300 FT MSL. WE NOTICED LATER OUR INS'S WERE ABOUT 3 MI IN ERROR. THIS IS WITHIN TOLERANCE FOR THIS FLT FROM MEM TO LAX (3 HRS 20 MINS). THE ERROR WE MADE WAS TRUSTING THE INS AND NOT XCHKING DME RAW DATA. WE WERE VERY HVY AND WANTED TO GET DOWN. THAT WAS OUR 5TH INTERMITTENT LETDOWN; ALONG WITH MULTIPLE FREQ CHANGES AND 2 CHKLISTS TO COVER. I HAVE SHOT THIS MULTIPLE TIMES BEFORE BUT NOT RECENTLY. IN MY OPINION; THIS WOULD BE MUCH SAFER BY NOT HAVING SO MANY LEVELOFFS AND ALL THE COCKPIT VERBIAGE AND COORD THAT IS REQUIRED.