Intermitant heat i can get heat out of my 2004 pontiac montana but it is not hot and sometimes when u have it on floor and defrost it will blow cold out of defrost and warm out of floor I have checked the coolant level it is fine and no leaks anywhere and no coolant smells inside the vehicle.

check the controller an vacuum lines

Is your heater blower motor working on all speeds ? Are you get any air coming from the rear vents ?

posssible air ducts not attached

Sounds like the blend door motor isn't working.

make sure coolant is in radiator....if its full, thermostat may need to be changed.....if thats not it check heater coil

your radiator may have frozen the thermostat

Warm up vehicle, then access heater pipes that run to rear of vehicle.  they should be HOT. if not you have  a circulation issue.  IF they are hot, then the blend door is not in the correct position.  this door directs air across a hot or cold core to create the correct air temp. check and update me.  

There is a mix damper inside your duct that controls the heat over heater core and ac evaporator that to your floor and one to your defrost. it is probable an electric actuator, that is not functioning properly. If yu can locate it and manually move it, you might have to remove the actuator motor first, you should be able to figure out your problem, I hope this helps! merry Christmas!

Try checking the cabin air filter, if equiped. It may be clogged making it draw more cold fresh air. Check your owner's manual to see if you have one or not, and it's location.

sounds like a stuck thermostat. but check if the system has a bleeder screw if it does burp it and that will help also.