I have a goodman package 3 ton unit. Everything works well except when it reaches desired temp the heat turns off and in the cool down mode it keeps running longer than it should and blowing cold air from the ducts till the temp drops again. Maybe every third or forth cycle. What could cause this. Thanks

When the temps get cold outside and even more so in damper weather... the coil in the outside unit gets iced up and must be defrosted. So the unit temporarily uses heat from inside the house to unthaw it and in the process leaves some cold air in the house.