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Hello and welcome to JA!!!
I'm Billy and i will help you with this.
Since you performed the pressure test on the vehicle, the only thing left that could be leaking, should be the radiator cap.
If the cap is leaking at high pressure, it will do so on long hills where the engine is loaded and heating.
In that case the slow leak, will get the car overheating gradually.
So the cap is the first thing i would replace for now.
Now you are right about the water pump giving same symptoms, but i should definetly change the cap first (much cheaper and easiest to install).
Now if when you replace the cap, you still have the same problem, there are many things to do, but i would start by having engine and radiator back flushed until water coming out is clear and clean.
But for now, change the radiator cap, and you will most probably get your fix.