crazed4macs

Im also having a similar issue as this thread here. Car does not overheat at idle, fan is working. Temperature beings to rise after about 20 minutes of highway driving. Flip the climate control to heat and engine temperature returns to normal. I've pressure tested the system and carbon monoxide tested for head gasket. Holds pressure and no exhaust is leaking into the coolant. Started with thermostat, changed out with 170 degree oe replacement. Problem persists. Coolant resevoir seems to get low rather quickly, and I have hot heat. Water pump? Any suggestions would be awesome. Also a 98 ex.