Why does your 1.6 v reg zafira lose all power when going over 70 on a motor way for over 40 mins turn it off and it woorks fine for another few miles but if you go under 60 it works fine all the time?

Why does your 1.6 v reg zafira lose all power when going over 70 on a motor way for over 40 mins turn it off and it woorks fine for another few miles but if you go under 60 it works fine all the time?
it is proberly restricted so if you go over 70 it will lose power but that only on vans usually