Hi have a '98 Sable, 3.0L, overhead valve engine. It's generally around 35 degrees F outside right now. If I start up my car and letit sit there in park, the engine heats up and the temperature gauge goes up to the 1/3 point or so and stays there, seeming to indicate the thermostat is doing its job, and I get heat out of the registers in the passenger compartment. If I then go driving down the road at 40 MPH or so, the registers stop putting out heat and start putting out cold air. Any ideas on what would cause this? I thought that probably the heater core was partially blocked like it was a year or two ago (and maybe it is), but my mechanic just flushed it out last week with no noticeable change. I then thought that maybe the water pump was only marginally working since there were a lot of problems with the water pump, corrosion, etc, so I replaced that. The old one didn't look brand new, but didn't look terrible, either. No noticeable change. (It was not a complete waste; as part of the process, I found that the tensioner was stuck, so I replaced that). Any insight would be much appreciated. Thanks!