B737-400 MISSES XING RESTR INTO PIT. BLAMES SOFTWARE AND TAILWINDS.

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WE WERE INBOUND TO PIT FROM S AND HAD APPROX 90 KT TAILWIND. WE WERE LEVEL CRUISE AT FL310 AND APPROX 130 NM FROM IHD AND GIVEN XING RESTR IHD/100 DME AT FL240. WE ACKNOWLEDGED AND ENTERED THE 'NEW' FIX INTO FMC. HOWEVER; THIS VERSION OF THE SOFTWARE WOULD NOT LET US ENTER THE RESTR ALT FL240. AS I WAS PNF; I TRIED SEVERAL WAYS TO ENTER THE ALT AND DID NOT 'MONITOR' OUR DISTANCE TO THE FIX AS I SHOULD (USUALLY) DO. BECAUSE OF OUR TAILWIND THE DISTANCE CLOSED RAPIDLY. ATC CALLED AT ABOUT 10-12 MI FROM THE FIX AND ASKED IF WE WERE GOING TO 'MAKE THE RESTR.' I RESPONDED THAT IT WOULD BE CLOSE AND ASKED FO TO USE SPD BRAKE AND 'DUMP' THE ACFT. ATC ADVISED TO 'CALL OUT OF FL280' FOR TFC; WHICH WAS NO PROB. BUT WE WERE ONLY FL250 DSNDING AT 100 IHD. I DON'T KNOW WHY THIS SOFTWARE PROB CAN'T BE FIXED. COST? BUT I KNOW IT HAS CAUSED PROBS BEFORE AND WILL AGAIN UNTIL IT IS UPDATED TO ALLOW ALT ENTRIES ON THE CRUISE LINE. CONSTANT MONITORING OF POS IS NECESSARY; POSSIBLE NORMALLY DONE; BUT I LET MY GUARD DOWN AND THE ABNORMAL TAILWIND AGGRAVATED THE SIT.