Overheating issue, I don't understand???

Hello,
I did a quick search on overheating, but didn't see what I was looking for, so I thought I would ask. I have a 1990 Bonneville SSI, 3.6 liter. Car always ran great, then sometime back, when it was really cold, my car started to overheat. I thought the thrmo was frozen. When it warmed up a bit, I installed a new OEM thermo, but I still have the problem. The really weird thing is that it does not act like any other overheating car I've ever seen. Here is what I am experiencing, Car get hot, but heater just blows cold air. Get on the highway, and the car WILL overheat, but I can drive for days on city streets at low speed. It will still get hot, but not overheat. This is totally backwards from anything I've ever seen....?? I then went under the hood. Pulled the hoses, one at a time, from each side of the radiator, and then from each side of the heater core. There was water flowing on both sides of the radiator, and the heater core. I thought the water pump was the culprit, but after doing this I thought maybe not... One thing worth noting is that I can be driving along, and the car is VERY hot, but blowing cold air out of the heater, then the car will start to make a kind of whining noise, and the temp will drop RAPIDLY, and heater will begin to blow hot air??? If anyone has any ideas of what the problem could be, please chim in and give me some advice. I really hate to tear into everything to change the water pump, if it is not the problem. I appreciate it!!!

NIBS

Off the top of my head, i'm pretty stumped.. But I do know a few guys that just might know ( I work for napa auto parts ) I would think to check some sort of relay or sensor for the cooling system on the car...

Where is the whining noise sounds like the belt.

Is the temp guage in the car actually saying overheat ?? Or is it actually overheating as to where it is boiling over ??.. If the guage is saying overheat and it actually is not over heating from being visible from under the hood, there might be a problem with the main computer... If so start to check sensors dealing with the cooling system... If this is not the case, run the car till hot.. When it is hot check around the radiator, hoses for even heat.. What I mean is if your top rad hose is hot and the lower is cold, something must be clogged in the system... Best of luck...

Due to you saying that the air is blowing COLD air it is leading me to believe that you MIGHT a) have a problem with your heater core or B) somehow you are getting a clog in your cooling hoses... Other than that I have no idea