I have heard of the same thing happening if there is a pinhole in the heater core. How's the cooling system? Has it been serviced or pressure tested recently? 
- Jim
Our yardbirds:
Belvedere, our '96 Jeep Cherokee, 4.0L ATX, 85,000 and going strong
Spencer, our '03 Chrysler PT Crusier, 2.4L ATX, 28,000 and just getting started
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