slow downs when resuming and saving flights

Has anyone else noticed occasionally when saving a flight or resuming one the speed of the aircraft slow down so much the plane actually stalls. Does anyone know what may be causing this problem?I've lost a flight like this because i resumed a flight and walked away and the plane stalled and crashed, vnav and lnav was enabled when flight was saved.Can't figure this one out, is it related to weather or the save function or VNAV not working when resuming a flight?????Thanks, any feedback