Chevrolet: cooling system 96 corsica, temp gauge, computer controled

Question
My brother has a 96 corsica 3100 v6 that keeps over heating, When you first start it you have no heatwhile it is warming up. the temp gauge will go anywhere from half way to all the way to hotthen all of the sudden you have hot air and the temp gauge will drop then after a few minutes you have no heat then it will overheat again.
then it will drop and do the same thing all over it has a brand new radiator we have put two thermostats in it (we were told that it is possable to get a bad one from the factory)
as far as i can tell all the air is out of it and it is full of antfreeze. does it have a vacum or computer controled valve that shuts the water off to the heater core and if so can it cause these symptoms?  

Answer
HI Nathan;
          Hi you have a new rad. was everything done right ? When you say it is overheating is it smoking ,boiling over real hot or just the gauge is showing it is hot ? I was just wondering if the gauge is off.
OK when you have it running are any heater  / water hoses getting sucked close ? and did you check the rad. cap if it is bad  it can cause you trouble.And there  could be a vacume heater valve in line on one of the two hoses that run back to the heater core , if so check it to see if it is working  as all of the above could be causing you problums.  and yes you could have picked up a bad radiator.  "" Good Luck ""