Mitsubishi Repair: Eclipse heating concern, coolant fluid, coolant level

Question
03 Eclipse 2.4 auto
Heater warms OK at startup, when driving heater works great but when stopped at red light for a time heater starts to blow cold air then returns to hot air upon excelerating.  

Answer
Gary,
It could be that there is weak coolant fluid pressure due to a worn water pump.  While you are driving, the water pump moves the coolant through your engine quicker than when you are at idle.  But a faulty water pump is not too common on a car that is only 2 years old; unless you have over 60,000 miles on it.  It's also possible there are air bubbles in your coolant system.  This can cause a blockage in your heater core and prevent coolant from flowing properly; which can cause cold air to blow.

Another thing to check is your coolant level.  After you turn off your engine, let it cool down completely.  Then, remove your radiator cap and gently squeeze your radiator hose.  If you notice coolant in the hose, you should be ok.  Also check the overflow coolant reserve tank to be sure it's at the proper level.  There are markings on the tank that will tell you when it's full.  

But if you do not feel comfortable checking these things, your local mechanic should be able to inspect it in a short amount of time.  
Good luck!