[islescar]

I hate to be the bearer of bad news. I have the same problem with my 98 sable. Heater core is clogged up tight. I’m guessing you have the 3.O Vulcan engine, or in other words, the weaker of the two engine options. These cars are notorious for rusty, thick, radiator fluid problems. This is because the water jackets rust out on these cars unless they are flushed religiously. For the sable the heater core replacement will run you $600 bucks. Because I'm a poor college student, every 6 months I disconnect the heater core hoses and run compressed air through it then flush the system. This may give you some heat, but is not a sure fix. With my car, my heat works fine when I'm on the gas pedal. When I come to a red light, I loose the heat. As my heat progressively gets worse, I know its time to flush again. I first looked into the problem when I bought my car a year and half ago and all I had was cold AC like you describe.