B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 477; Flight Crew Type 12400
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Flight Crew FLC Overrode Automation
Aircraft
Aircraft with new FMC software installed. While cruising at FL390 and about .79 Mach setting up for the Arrival doing our Descent Briefing; ATC cleared us to descend to FL330 then cross about 20 west of ZZZZZ intersection at FL240. FO selected FL240 on the MCP ALT window then built the fix 20 west of ZZZZZ intersection at FL240 in the FMC; I verified it; and he executed it.We had plenty of room for a cruise descent so he then hit ALT INT to initiate the cruise descent requested by ATC. We resumed our brief. Instead of descending at the current cruise Mach; the aircraft initiated a deceleration to about .63 Mach just above our low speed warning window. We paused the brief; recognizing that this was not a desired descent profile and switched to 1000 ft from vertical speed descent with the current .78 Mach in the speed window and investigated.The FMC cruise altitude was now at FL240. We modified the FMC cruise altitude to the desired FL330 intermediate level off then reengaged VNAV. The aircraft continued descent as expected at the slightly lower Mach speed for FL330. I have never seen an aircraft reset the desired cruise altitude due to hitting the ALT INT for descent. If this is an error with the new FMC software this could easily lead to an unexpected high altitude low speed condition.
B737 Captain reported that new FMC software may have been responsible for the aircraft slowing excessively when ALT INT was selected to initiate a cruise descent.