ACR MLG ALT DEVIATION UNDERSHOT ALT CROSSING RESTRICTION DURING DESCENT.

Narrative

FLT WAS ASSIGNED A XING RESTRICTION OF 35 SW OF BLD VORTAC AT 15000'. I PROGRAMMED THE FMC (FLT MGMNT COMPUTER) FOR THIS XING RESTRICTION AND VERIFIED THIS INFO ON THE FMC CDV. THE COPLT ALSO CONFIRMED THE CORRECT DATA ENTRY. AT APPROX 35.5 DME SW OF BLD VORTAC; LAX CENTER ASKED US OUR DME FROM BLD. (NOTE: THE FMC INDICATED 42 MI SW OF BLD; IE; 6 MI FROM THE FIX.) WE TOLD THE CTLR TO STANDBY SO WE COULD VERIFY THE DME DISTANCE WITH RAW DATA DIRECTLY FROM THE NAVIGATION RECEIVER. UPON ACCOMPLISHING THIS; THE DME INDICATED WAS ACTUALLY 35.5 MI; A DISCREPANCY OF 6.5 MI FROM WHAT THE FMC WAS INDICATING. WE TOLD THE CTLR WE WERE 35.5 DME FROM BLD. THE CTLR THEN ASKED US OUR ALT. WE RESPONDED; 'WE ARE PASSING THROUGH 18000' (THE CTLR THEN HANDED US OFF TO LAS VEGAS APCH). THE COPLT AND I BELIEVE THE PROB WAS CAUSED BY THE COMPUTER MALFUNCTIONING. THE TRIP HAD ORIGINATED IN ONTARIO; CA. IT TOOK THE COPLT AND I 1 HR TO ALIGN THE IRS SYS. WE BELIEVE THAT THIS DIFFICULTY COULD HAVE HAD AN EFFECT ON THE ERRONEOUS DME INFO THE COMPUTER WAS GENERATING. IN SUMMARY; THE COPLT AND I BELIEVE THAT COMPUTER MALFUNCTION WAS RESPONSIBLE FOR THE MISSED XING RESTRICTION. HOWEVER; WE HAVE LEARNED THAT BACKING UP THE COMPUTER WITH RAW DATA FROM THE NAV RECEIVER COULD PREVENT AN OCCURRENCE OF THIS SORT IN THE FUTURE. INCIDENTALLY; FLT DID NOT POSE ANY CONFLICT TO OTHER ACFT AS A RESULT OF THIS ERROR AND WE WERE NOT ASKED TO CONTACT ANYONE WITH REGARD TO THIS INCIDENT.