This usually happens when I'm above 32 thousand feet, the autothrottle knob turns into 0 and I have to manually put it back to the desired speed, once it's close to the speed it does the same. The behavior is easily reproducible, just set the plane over 32 thousand feet, and set the desired climb speed in 0,84 Mach with FLCH selected and LNAV.
This usually happens when I'm above 32 thousand feet, the autothrottle knob turns into 0 and I have to manually put it back to the desired speed, once it's close to the speed it does the same. The behavior is easily reproducible, just set the plane over 32 thousand feet, and set the desired climb speed in 0,84 Mach with FLCH selected and LNAV.