2001 Status Please HELP!!!!

Hello. I have a 2001 Dodge Stratus 2.4L 4cyl. This car has had a lot of problems; I just had the radiator replaced last month because of a major leak. Now it is throwing the error code for Oil Pump Positioning Sensor, the dealer that gave me this information said none of their mechanics had ever heard of this...go figure. Anyway, the temperature gauge was in the red and the light was coming on, that is why I had the car checked for error codes to begin with. A nice gentleman from Autozone replaced the thermostat and the radiator cap for me and this seemed to cure the problem, but not for long. It is still overheating when I drive more than 30 miles or so and the antifreeze in the reservoir boils when I turn the car off. There is no water in the oil, so I am hoping my head gasket is ok. Does anyone have any suggestions as to what the problem might be? The waterpump maybe? The dealer suggested that I might have an air pocket somewhere within the radiator, he said that sometimes happens after the radiator is replaced....?

if it's an air pocket, which i doubt, that can be fixed by bleeding the system... but i'd probablly say you have a defective water pump

Thanks for your input, I am going to try to bleed the air out today.