Failure to engage, I can certainly recall. But 'random disconnects'? Very, very unusual.
The best autopilot snag I ever read was on one of the Crows' F-4s we were flying at the time: Use Autopilot with caution - prone to random uncommanded +4G pitch errors. But I don't think that I ever used the autopilot in my brief times on either the Buccaneer or F-4 though - there was simply no point.
During my brief acquaintance trip on an A330, the instructor disconnected the AP....and absolutely NOTHING changed - the aircraft maintained its trimmed state until I deflected the sidestick. It took about half a minute for me to understand how simple the manoeuvre demand character of the Airbus flight control system made flying the aircraft.