How about this...
...there have been several incidents regarding buffet alert bla bla bla....
could be possible that during FMC programing the temperature expected at a certain FL has not be put correctly or not at all? Dunno why 738 FMC do not make this entry mandatory. The fact is that if it is not entered, the FMC will assume standard temperature making it -61 at FL 380 . This never happens most of the time as we are at ISA +10 to + 15 depending on the place on earth you are flying and in what season. Not entering the correct temp you will get on the CRZ page OPT/MAX maybe 365/382 but once you enter the correct data...SURPRICE!! On the 744 this entry is mandatory FMC has boxes and not dashes as on the 738 FMC. Summer season has been specially hot this year. Here in Asia we had some times ISA + 20 over Japan airspace.
Hope this helps... if not we have to call Spiderman to solve the mystery
Happy landings...