Okay I've been fighting with my 2000 X for months now. I took it into my Nissan dealer and had them run a full diagnosis on it. It came back that my radiator and lower hose needed to be replaced, costing about $850. I did some research on here and looked at how difficult it would be to do the work myself. Found all the parts online, and replaced the radiator, upper and lower hose, all of the hose clamps, the thermostat and the temperature sensor. I figured it would be easier to replace anything that could have gone bad with the couple overheatings I had encountered. Put all new coolant in, and bled the system multiple times in order to ensure that I do not have any air left in my system. I drove to our lakehouse this past weekend, and stayed at a normal temp until exiting off the highway when my car overheated. I drove on the highway for about an hour, going between 55 and 75 mph. We thought there might have been air in the system still, so I went through the bleeding process again. Same thing happened on my way home from the lake. I can drive all around the cities going 60 mph to and from class with no issues. It only happened after driving on the freeway for an extended period of time. I asked Nissan to specifically check my head gasket, they said they tested the pressure and it was fine. I also am leaking a small amount of coolant still, while all of my hose connections are dry. So I have kinda limited it down to the fan clutch or the water pump, but I am not exactly well versed on cars or know much of what I am talking about except for the research I have done. Does anyone else have any ideas? Everyone I have talked to isn't really sure of what is wrong. I appreciate any help that I can get. Thanks so much!