From past experience I will say PAY THE MAN! It really is a P.I.A. job. We had similar situation in a 97 w/ a 4cyl. without the slime, luckily it was fixed by just flushing about 2lbs. of gunk out of the cooling system.

In your case, I think it is the heater core due to the slime on the windshield, and leak in floorboard. You didn't say but I am assuming that the amount of air flowing out of the defroster vents hasn't changed. I suspect you may also have a partial blockage which would explain the gradual loss of heat.

Good Luck! <img src="/forums/images/graemlins/cheers.gif" alt="" />