2006 Buick Lucerne heat

When you engine speeds up the water pump is working faster to push the coolant through. You may have a blockage in your heater core so only higher pressure can go through. Back flushing the heater core is a good start. Disconnect both hoses and flush it with a garden hose until you think you have all the crap out. The top up with new coolant and try. Flushing the whole system may not be a bad idea if you have never done that yet.