737 Autopilot seems to just stop working

2 of my last 3 flights in the 737-900, as I tried to slow the plane down with the A/T engaged, it seemed to just fail. It worked FINE through the entire flight, but trying to slow from 250 to 140 KIAS, it ceased to function. Sometimes turning the A/T off & back on again seems to fix the problem. Naturally, this is an extremely difficult problem to contend with when trying to land the aircraft. Has anyone else had this problem? Any ideas?Chris Catalano