Heater blowing cold

Hello, I have an '03 Jetta. With the thermostat all the way up on hot, the heater blows cold air unless I'm cruising down the highway. Then it blows very hot. If I exit or hit slow traffic, it will blow cold again.
I'll chek collant level when I get home, but does this sound like anything besides low collant? The temp gauge shows warm up and running temp normal.