Nelsonea good morning! As there are few places coolant can really go without leaving signs except for internally it can evaporate due to excessive temperature. your over heating after highway speeds could cause a loss but not excessive loss. There are a few things to try so that your not just changing parts to eliminate options. Two very strong ideas are what you already mentioned, the pump and fan. Both your ideas can be easily checked in the driveway.

Start the X and get her to running temp, top her off, set the A/C to max, get a cold beverage and wait! With highway driving your pushing ample amounts of air through the rad and cooling through basic properties of forced air and pumped coolant, then at the point of slowing your system then relies upon the fan to provide that same air and pump to cool the engine down but if either one or both is not up to par the system will fight to recover from the load and then not exactly over heat but run in excess of the norm or yes over heat. So by allowing the X to sit and run for an extended amount of time at idle and then revving the engine to say 2500-3000 RPM's you can rule out both of these possibilities.

The fan should not just spin freely with the X off (clutched/therm), there should be resistance if(clutched) not really if elec. and with therm yes/no. Clutched/Therm models should change volume as in tone or loudness as the temperature rises when running or as the ideal rises it should spin faster as the clutch engages with the higher RPM's. (Robb a Ford Ranger of its Elect. Fan system, say 05-12 at your local salvage yard. The complete elec. system on those is simple and will fit. one less question because when its bad you know it, it just simply doesnt come on at between 199-210 degrees!)lol

At running temps and with good circulation the hoses should have resistance from the pump creating pressure, both upper and lower hoses. They should also be hot not one hot and the other mildly hot/warm. At running temp your heat should be hot with the blower at max, if the heat lowers or becomes cool this also is a sign the pump isn't up to par and or there is a blockage.

Head gaskets can be tricky as they can be bad and still show good pressure however if shes burning antifreeze due to a gasket the plugs and exhaust are your best place to research that issue as the plugs will collect residue and the exhaust will smoke a little and or smell sweat on occasion. The performance of your engines idle, acceleration and MPG can suffer greatly with a bad head gasket showing good pressure readings. There are to many variables involved to just state oh the pressure is good so the gaskets are good. I Wish it was as easy as that!

Sorry to run on, I hope this helps!