So why does the FMC reprogram itself for no apparent reason.  I was doing a flight from KSAN to KSFO and suddenly my speeds where different than what I had programmed.  the speed was 180 in small case even though it kept flying at 292 kts.  But I had no T/D so I had to manually descend until my computer locked up.  Why does it change the settings?  I don't understand yet.