Why does the heat in my car only get hot when i mash on the gas hard?

It is a vacum problem.

Your controls are all on vacum lines.

Check the lines under the dash.

- ALSO TRY BLEEDING SYSTEM, LOW COOLANT MAY CAUSE "SHUFFLING". COOLANT BYPASSES THE HEATER CORE UNDER LOW FLOW CONDITIONS. INCREASE SPEED WILL CAUSE HIGHER VOLUME TO CORE, CAUSING HEAT WHILE UNDER ACCELERATION ONLY.