Looking at the picture.
You have the older style radiater header tank(painted black). Some time the paint on the inside flakes off and then circulates within the engine and causes weird issues.
You have the older style radiator. The newer ones have a cap on the radiator itself.
You really need to get a pressure test done on the system by a radiator shop. They replace the cap on the header with a tool and then pressureize the system and look for leaks.
Some times the hoses only leak out under high pressure( you may have a pin prick).
Try the flush first then get a pressure test.
Car should be cold
1. Drain out old coolant
2. Fill coolant via header tank slowly (to prevent air locks) approx fill + 7 l
3. put rad cap on
4. turn heater on full hot. run interrior fans
5. start engine. run engine at +2000 -2500 rpm until rad fans cycle twice
6. Switch off engine, wait for it to cool
7. remove cap. turn cap slowly to release pressure(becareful not to burn yourself)
8. start engine fill header tank
9 check if top hose is full of water. press it
10 put rad cap back on. drive it around the block
11 repeat 6,7,8
12 it may need a few more bleeds to get all the air out
Alternate way to get out the air (note you must have already put in the coolant)
1 remove header tank raditor cap.
2. run engine at fast idle
3. slowly fill water in header tank.
4. massage top radiator hose (as you press it in and release it(the action sucks the water into the radiator)
5 water will bubble out of the top of the header tank. (burp) do this until all the water stops flowing out.
Note you must have the heater on to bleed all the air outta the system.
These old style rads at a pain to get all the air out.