Clogged Radiator will block the coolant flow mostly at highway speed, more
air resistance, more heat, less flow, more pressure.

He said that the system was fine before, he did something wrong in between.

What am curious to know, did you play with the thermostat wile bleeding or
changing the exp tank?

Let us know why the thermostat had a broken seal, your answer could help others.

Cheers
Ben
http://www.benplace.com