1991 Dodge Spirit Radiator Fluid

I have a 1991 Dodge Spirit with 130,000 miles. I replaced the radiator about 3000 miles ago promptly after overheating. Now it is overheating again. Conditions: The is no fluid in the resevoir when the engine is cold. After running the engine for a mile down the road the heat spikes and stays hot. When I check the radiator the resevoir is full. The fluid has brown bits in it and returns to the radiator when the engine cools again. The coolant was also flushed when the radiator was replaced. I was thinking it was either the thermostat or water pump is malfunctioning. Your imput is appreciated, as the nearest mechanic is 65 miles away.