wierd question please help

[eclipsegt123]

Hi there i have a 2003 f350 6 liter and it has a brand new head gasket and studs on it. What my problem is i live where is is cold -25c and i find when im driving and i stomp on it the heat goes really hot and then when i let off it blows cool. My anti freeze is full and i have card board in fron of the rad and the temp is right where it should be. What else could it be

[MrWilkerson]

Sounds like your heater core is stopped up or maybe a collapsed hose?

But here's a for ya!