I have the 3.8 engine and mines was recently doing this. It didn't fix itself until I put coolant directly in the radiator then blast the heat for like 5 mins. Idk if this apllies here seeing as though you have the 3.4 hopefully it helps

 

We always put rad fluid straight into the rad. thats where your supposed to put it to get proper amounts in it :-)
Bought more fluid and topped it off. i now have heat. wasnt more than a half gallon and there still could have been an air pocket in the system as well. BUT at least its better now thats all i can say!
really glad because im tire of changing, swapping, buying parts! lol