My 04 Chevy Venture heater doesn't get hot, I replaced the thermostat and it did not help. When at an idle it is rather cold, when driving, itgoes hot then cold - back and forth.

Usually it's a leaky thermostat allowing to much coolant to pass through it when it's closed.To check this, clamp off top rad hose completely(acting like a closed thermostat) If you have hot air from the heater now ,you do need a replacement thermostat.

what makes you think it overheats.
the guage only?
or boiling over.

is A/C on or off, at over heat, assuming it really is???

Sounds like you need to have the cooling system flushed out then have them put a new themostat in it. If that dont fix the prob you might have to put a new heater core in it or radiator.

Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)

In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.

Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.

Replace the thermostat. the temperture should be at the 190 to 200 mark

It sounds like the coolant level is getting very low. Only check it when it is cold! Check the level in the radiator and in the overflow bottle. Chances are you have a leak somewhere, these vans had problems with intake manifold gaskets. A big job.

If this happens when you are idle, and then you get hot air as you drive, most likely you have low coolant and it only cycles as you drive. Get approved dextrom gmc coolat and cap off your coolant it is most likely low.

first of all the water pump doesn't have a heater, all the water pump does is cycle water (coolant) throughout the cooling system, the heater is up inside the dash behind the glove box.

I would feel the two 3/4" heater hoses that run from the engine into the heater core, if both of them are hot (providing the engine is at full temp) it one is hot and the other is cold maybe the heater core is plugged.

Chevy made some weak intake gaskets on 3.1 and 3.4. My guess is that the intake gasket is weak and/or leaking and it is letting pressure out somewhere. At idle maybe the system pressure is to little to push hot coolant into the heater core.

you should have hot water on all rad. hoses when the motor is running at a norm. temp. the water is not going through the heater core like it should. you have cutoff valve bad low water are a stopped heater.try a hoter thermostat if none of the above are true.