Chevrolet Repair: heating issues, 2006 chevy impala, coolant system

Question
I have a 2006 Chevy Impala 3.9.  A few months ago I noticed that my car would only blow warm air if I gave my vehicle gas and ran it above 35 mph.  If I come to a stop sign or light, it goes from warm air to cold in a blink of an eye.  Thought that because the resevoir was a little low, add some fluid and it should be better.  That didn't work.  The level on the resevoir has not moved but the car has gotten a little worse.  Checked the radiator and fluid was low in there.  Added fluid there, even though the resevoir was fine and that didn't seem to do anything.  Now my car is overheating and throwing antifreeze from somewhere.  I have been told a couple of different things and just trying to figure out what might be wrong.  If I have fluid in the resevoir, why isn't it going into the radiator if that is low?  Why did it only blow warm air when I was on the gas and cold air if we were idling?  Thank you so much for your time and expertise.

Answer
MORNG AJ, WELL LET ME ANSWER THE FIRST QUESTION AS WHY THE FLUID FROM THE RESERVOIR WILL NOT GO TO THE RAD AS FAST AS IT SHOULD..IT TAKES TIMES FOR THIS TO HAPPEN BECAUSE IT IS A CLOSED SYSTEM..THIS MEANS THE RADIATOR NEEDS TO BE FILLED TO THE TOP WHEN HOT,THEN AS THE RAD DROPS DOWN IT WILL TAKE COOLANT FROM THE RESERVOIR AS NEEDED..SO BEFORE THE SYSTEM WORKS CORREECTLY THE RAD HAS TO BE FULL [AND NO LEAKS IN THE SYSTEM,REMEMBER CLOSED SYSTEM]..NOW AS TO WHY THE HEAT WOULD ONLY WORK UNDER ACCELERATION IS BECAUSE THE SYSTEM IS OR WAS LOW,SO ALL OF THE NEEDED COOLANT WAS IN THE ENG BLOCK AND RAD THE HEATER CORE IS THE LAST PLACE TO GET COOLANT. ONCE YOU GAVE IT GAS IT WOULD PUSH THE HOT COOLANT INTO THE HEATER CORE AND THEN DEPLETE ITSELF ONCE AGAIN AT IDLE..AGAIN THIS SYSTEM NEEDS TO BE FULL ALL THE TIME..NOW AS TO WHAT YOUR PROBLEM IS YOU HAVE TO TAKE THIS CAR TO A SHOP THAT CAN PRESSURIZE THE COOLANT SYSTEM AND CK FOR A LEAKS..THIS IS TOPS ON THE LIST.UNLESS YOU KNOW WHERE IT IS LEAKING AND YOU CAN FIX THIS YOURSELF. ONCE YOU FIND AND [REPAIR YOUR LEAK] YOUR HEAT ISSUES WILL MORE THEN LIKELY BE TAKEN CARE OF..LET ME KNOW HOW THIS GOES AND GOOD LUCK TO YOU. TOM..