maximum engine temp

What kind of thermostat? a typical parts store poppet thing? I might suggest a balanced performance one, Mr Gasket makes some, some companies like RobertShaw and NAPA have them too. Sounds like it may be closing off or down a little under higher rpms. You could also try gutting a stat and see if the highway temp stays down. From 180 to 130 isnt too much really, for a 4 core brass unit. But it may be limited by how much water the Tstat is letting by.