Okay so regarding the questions you have asked...

I know what conditions my car was originally overheating....whenever the engine would heat to normal operating temperature and the radiator was supposed to kick on - it would not work and result in overheating. In order for my X to overheat now, I have to be driving on the highway or freeway for an extended period of time. Once I exit the freeway, and only after I exit it, will it overheat.

I replaced pretty much everything in the cooling system instead of just the radiator and lower hose because the other components had not been changed out previously, and I wanted to just do it all at once since the extra parts did not cost much more.

Of the coolant I have noticed missing, it has ended up on my driveway or garage. It is very inconsistent in which conditions it will leak, but I know that it is leaking out and endeding up on the ground. I have checked all of the hose connections, and all are dry so I am sure they are not leaking anything. The slight loss of coolant does not correlate to when my car has overheated, since I have noticed small puddles on my driveway long after the overheating occured.

Upon replacing the radiator, I also installed a new radiator cap.

The timing belt went out about 25,000 miles ago, and therefore that was just recently done. Our old mechanic (who did the timing belt) did not document very well and I am trying to get in contact with him to see if he replaced the water pump at that time.

I have always used a 50/50 mixture of antifreeze and water.

I am not really sure about the heater core...or how to tell if it is bad or if something is wrong with it.

I did an oil change last week to check if there was anything in it, and it was all clean.