Hello and welome to Just answer.com, Super mechanic here. About your vehicle.
There have been a few sevice bulletins that have to do with the coolant almost becoming like mud. You might want to have the radiator boiled out. Here is what the bulletin says.
File In Section: 06 - Engine/Propulsion System
Bulletin No.: 99-06-02-012D
Date: April, 2002
This bulletin is being revised to include additional model years. Please discard Corporate Bulletin Number 99-06-02-012C (Section 06 - Engine/Propulsion System).
Some customers may comment that the heater is inoperative, blows cold air, engine may exhibit an overheat condition or the coolant reservoir has rust in it.
Vehicles equipped with DEX-COOL(R) coolant, which may have been operated for extended periods of time with a low coolant level, usually in excess of 32,000 km (20,000 mi), may be susceptible to the formation of a rust-like material in the cooling system.