In the winter the cars heat stopped working for 2 days before now working again, Then this summer the heat only works at 3,4 but not 1,2. It then began to overheat over the red line, turns out the coolant reservoir was empty but the rad still full. Took it and had a coolant pressure test and no leaks could be found. All the clamps were tightened and the rad cap replaced as well as a coolant flush. That was good for about 2 months. Then the car ran out of coolant again and I have had to add a little bit in every 3 weeks or so. Recently it has been running hot in stop and go traffic, but has not passed over the red line, always stays under. 3 months ago after the initial pressure test the technician told me they could not find anything but that it could potentially be the thermostat.
Any Ideas? Cannot really see a leak under the car, even now.

ALSO, when I change the oil it is just black, is NOT hot chocolate colour or swirly, I do not think coolant is mixed in, so I doubt it is the headgasket?

Any help would be appreciated.
THANKS
Water pump is only 10 000km old
Car has 290 000km
Can I Touch the radiator right away with my bare hands quickly when the engine was just shut off?
Also, I do believe the rad is fairly new, They should have diagnosed a clog when doing pressure test and tightening clamps, rad cap, etc.