07 Malibu Intermittent Heat??

2007 Malibu, heat only blows warm during acceleration, when you come to stop/idle heat blows cool. Temp gauge remains constant in normal operating range while driving. Thoughts?

First thing I would do is, check the coolant level, on a cold engine....

I second TechII. Heater works off the heated coolant. If there isnt enough coolant to completely fill all the lines, IE heater lines, Then it isnt going to blow heat. Check the color of the coolant as well. If it is low, discolored, or has foreign debris, you need to change it (yes, very soon). Even if its low, change it, the chemistry in it would have degraded, and it would have turned acidic, and yes acid loves to eat gaskets. Thats the problem with OAT or Dexcool type coolants.