heater doesn't work properly on 2007 Saturn Relay

The front heater will blow hot but when you stop for a light it will start blowing cold but will again blow hot when you speed up and the rear heater never blows hot. What could cause this problem?

2 answers 2 comments
MAKE SURE IT IS FULL OF COOLANT. HOW MANY MILES ON ODOMETER?
It is full of coolant. I even took the cooling system cap off and added water, which was only about a cup full at the most. The van has approx. 93000 miles on it.
sounds like coolant level is low ck that 1st thing
I checked by removing the cooling system cap and added water to the system there, which only held about a cupful at the most.