irregular engine temperature

I have a '98 Venture that has already had the intake and head gaskets replaced at 130,000. It has 180,000 miles know and it recently started running hot, then cold, then hot. Temp. guage goes up and down very fast and irregular. Also when it goes to hot I get no hot air out of the heater inside. I put in a new thermostat and thought it fixed it, but after running a little longer still have the same problem. I have used both bleaders in the system to make sure I have the air out of the system as well and no luck fixing problem. If anyone has any ideas please let me know..:banghead:

Thanks

are you loosing any coolant? if so then the erratic temp hand is a symtom of a leaking intake gasket.any white gunk on the bottom of the oil cap? is the exhaust hot enough that you can't hold your hand closer than 2"? one more thing was the t-stat made in mexico? as these will not open even in boiling water.

I don't seem to be loosing any coolant, no gunk on cap, haven't tried the exhaust heat yet, do you mean at the tailpipe or by the manifold? No white smoke out the exhuast either. I'm going to try to have a chemical test done on it. Stat was made in Indiana, it is a Stant.

if you hold your hand 2" from the end of the tailpipe while it is idleing if there is any steam in the exhaust you won't be able to hold your hand there long. if the exhaust from the tailpipe makes you move your hand then the motor is burning the coolant. if the exhaust is warm then there is no steam in it.

YOu have all the symptoms of a blown head gasket again. Really unwelcome, I'd like any other explanation, but facts are facts, sorry.
And...each time you burn 1 gallon of fuel, 1.3 gallons of water goes out your tail pipe in addition to CO and CO2 as combustion byproduct...that is the law of chemistry
http://www.madsci.org/posts/archives/apr99/923373229.Ch.r.html
http://www.avigex.ca/xport/watervapour.txt
...so it is obvious you will always have water/steam going out the exhaust....a lot more than a small coolant leak will provide.