my truck blows cold air until i start going about 55mph and then it blows normal but anytime i slow down it immediately goes back to cold. I changed the thermostat and the problem still exists. any suggestions?

A couple of thing could have happened the A/C heat blend door acutator may have become dislodged and the temp never really gets very hot. I and another Technician repair the Blend door by heating the plastic and bending it. Under warranty get the entire heater case replaced. But make sure the dealership requests special shipping of the new heater case because some times FED-EX really bangs up the box and breaks the new heater case during shipment.

Your heater core might be blocked somehow

Its called a mechanic....