It could be a couple things. First, when driving, the RPM's are higher causing the water pump to push more coolant. Make sure the system is full and there are no air blocks. With the engine cold, loosten the return hose from the heater core to see if air comes out.
Now, you said one of the heater hoses from the core stays cool. That tells me either the heater core is partially plugged or there is an air block. Disconnect the hoses and back flush the heater core with a garden hose to see how the water flows through it and how much dirt comes out.
Let me know what you find.
Saturday, September 10th, 2011 AT 4:52 AM