I had the exact same symptoms and problem and also solved with stop leak. I actually hated doing it, as that stuff can really mess stuff up. But it's a 99 with 150k and a $800 heater core replacement just isnt' worth it. Most feedback I found seemed to think the stop-leak would work for 2-3 years or more. I will be selling mine soon, and it has been working great since I did it about 6 months ago.
You said you don't have leaking anywhere, but you probably do... if you look under the passenger side compartment, way under the glove box, up to the front of the carpet... there is a heater vent right there. Pull the carpet down a bit from that, and you might discover your leak. it doesn't gush, but you'll see where it is running.
I agree, the worst part is the smell everytime the vents turn on. I think mine is finally starting to get evaporated so it isn't quite as bad. Inhaling those fumes forever can't be good for us