try mo check rotation ng auxiliary fan mo. pero base sa salaysay mo, most probable culprit is your water pump.
here's what to do.
1. run the car at more than 100 kph as you have stated, then pag lumampas na ng kalahati, subukan mo patayin ang air con while maintaining speed. observe the temperature reading
btw, di ba naman bumabago ang lamig ng aircon?

anyway, noted then na kahit lumamapas ng kalahati at nanatili doon, it is still tolerable, hwag lang umabot sa red line