I only did two flight with this new B767 and couldn't lock the A/T to either follow the speed mode (in mach as I'm talking about the cruise portion) or the VNAV mode. (Both modes set to M082.

In fact the reason of the "infinite acceleration and thus overspeed" comes from the fact that the A/T constantly goes for max cruise setting (displayed in green) which is way to much and will result in a overspeed.

BTW I tried it with the B757 on the same flight and I have the exact same issue.

Does it work for you Alex ?