Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem
Aircraft
Traveling in level cruise at FL370; smooth air; Mach .80 we experienced an aircraft Mach speed exceedence. With no noticeable wind change the aircraft accelerated; uncommanded; up to Mach .84. The A/T system made minimal; if any; changes to the cruise power setting. The Captain (PF) immediately spun the Mach speed way back and extended full speed brakes. This was the point at which the A/T finally brought the power back. We stabilized the aircraft; configured the automation and continued to the destination with no other abnormalities.We possibly could have reacted more aggressively and sooner; but we fully expected the A/T system to respond correctly to the speed increase and reduce power accordingly. Aside from that; I cannot see anything else that could have prevented the situation.I don't know what we could change to prevent this situation again.
Air carrier First Officer reported a Mach speed exceedance during cruise.  First Officer stated the increase in speed was uncommanded and there was no noticeable wind change either.