My Jetta is overheating. Started slowly at first, but now overheats in 5km.

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I have replaced all temp pickups and fan switch, thermostat (drilled 3mm holes X2), have replaced the reservoir tank. While thermostat cap was off, checked impeller, used torch and screwdriver to c if impeller is broken or loose (100%). NOT loosing water, no oil in water, no vapor out of exhaust. Put a manual over-ride switch on fan, temp keeps climbing. Took head off, checked (NOT blown gasket or cracked). Took all hoses off and forward and backward flushed everything, yes, heater on full heat. Took radiator out and flushed as well and inspected (100%). The system gets presurised, can feel it on the hoses can c in reservoir as well the water squirting. Asked a mechanic for advise, he checked everything as well, and said, "Hey pal, you have one hell of a problem. All my ideas are done." Please help, I've now spent what funds I had and still no solution.