FLC FAILED TO MAKE XING RESTR DURING DSCNT BY UNDERSHOOTING THE ALT.

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ALT XING RESTR. THIS WAS THE SECOND TRIP WITH COPLT WHO HAS APPROX 140 HRS IN THIS TYPE OF ACFT AND HAS A GOOD KNOWLEDGE OF THE FMS. WHILE OUT OF THE COCKPIT TO GO TO THE LAV; ATC CLRED US FROM FL410 TO FL330. COPLT STARTED DSCNT ON FMS AND INFORMED ME OF THE CLRNC ON MY RETURN. DURING DSCNT ATC CLRED US TO CROSS HARTY INTXN ON THE LENDY 4 ARR AT FL230. THE COPLT MADE AND INPUT TO THE FMS. I STARTED TO XCHK THE FMS AND REALIZED THE FMS/AUTOPLT WAS NOT GOING TO MAKE THE XING RESTR. I ADVISED THE COPLT OF THIS AND HE TOOK OVER MANUAL CTL OF THE AUTOPLT AND ALL INDICATIONS WERE GOOD THAT WE WOULD MAKE THE XING RESTR. WHEN THE AUTOPLT WENT INTO ALT CAPTURE IT REALLY FLATTENED OUT THE RATE OF DSCNT. WE HAD TO FORCE THE ACFT TO FL230 AND CROSSED HARTY ABOUT 700 FT HIGH. ATC DID NOT INDICATE ANY CONFLICT DUE TO OUR XING HARTY HIGH. WE DISCUSSED POSSIBLE FMS PROGRAMMING PROBS AND THE NEED TO MONITOR THE FMS/AUTOPLT AND TO MANUALLY CTL THE ACFT AND AUTOTHROTTLES IF THE ACFT IS NOT PERFORMING AS DESIRED.