"I have heard" that the US registered 767's have a higher Vmo/Mmo than the UK reg ones, due to different buffet margins in certification. I would love to know if this is true or not. The ones I fly are limited to Mach 0.84, but the link below says Mach 0.86.
Certainly at high altitude the speed doesn't seem that stable, but I put that down to the autothrottle. Without it, at a constant (correct) thrust setting, the speed stays very stable indeed.
This place
http://www.smartcockpit.com/plane/boeing/B767/ seems to have some useful information - although I would treat it with the usual prudence.