A B737-800 FMC DID NOT RECOGNIZE REACHING THE PROGRAMMED CRUISE ALT. AN AUTOMATIC FMC TOP OF CLIMB DSCNT COULD NOT BE INITIATED.

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POSSIBLE GENERIC PROGRAMMING BUG IN VNAV SYSTEM ASSOCIATED WITH TOP OF CLIMB. WHILE CRUISING AT FL390; WITH VNAV CRUISE ALT OF 39000 FT; ATTEMPTS WERE MADE TO ENTER A DSCNT CROSSING SPD AND ALT RESTR FOR CURSO INTXN (250/14000 FT). THE ATTEMPT WAS REJECTED BY THE FMC REPEATEDLY. HAVING SEEN THIS WHEN THE VNAV ALT WAS HIGHER THAN THE ACFT CRUISE ALT (FOR EXAMPLE WHEN RESTR BY ATC TO LESS THAN THE PLANNED CRUISE ALT); I VERIFIED THAT THE CRUISING ALT IN VNAV WAS THE SAME AS THE ACFT ALT (39000 FT). THERE WAS OCCASIONALLY A T/C POINT ON THE MAGENTA LINE; INDICATING THAT SOME PORTION OF THE VNAV DID NOT BELIEVE THAT WE HAD ACHIEVED OUR CRUISING ALT. I ENTERED A CRUISING ALT OF 38000 FT; THINKING THAT THE FMC WOULD CLEARLY RECOGNIZE THE WE'D REACHED OUR TOP OF CLB; BUT THAT FAILED TO RESOLVE THE MATTER. AFTER A MANUAL DSCNT (NON VNAV); AT SOME UNRECORDED ALT; THE VNAV ALLOWED ENTRY OF THE CROSSING SPEED AND ALT INFO. I DID NOT BELIEVE IT TO BE AN ACFT SPECIFIC PROB; SO DID NOT ENTER IT IN THE MAINT LOG. WE HAD FLOWN THIS ACFT FOR THE PRECEDING FIVE LEGS; AND HAD NOT EXPERIENCED THE PROB BEFORE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE CREW WAS WITHIN 100 MILES OF THE TOD AND THAT A HIGHER CRUISE ALTITUDE HAD NOT BEEN ENTERED INTO FMC CRUISE PAGE. ALL PARAMETERS THAT SHOULD HAVE ALLOWED A NORMAL TOP OF CLB DESCENT TO BEGIN WERE PRESENT BUT THE ACFT WOULD NOT DSND. THE CREW SELECTED V/S AND AFTER CAPTURING THE VERTICAL DESCENT PATH WERE ABLE TO PROGRAM AN FMS DSCNT WITH THE FIX CONSTRAINTS.