radiator is OK - no cold spots, nice temp decrease from inlet towards the outlet.
But I seem to have found the culprit - I had approx 70% (or more) anti-freeze there - so took 5 liters out, put 5 liters of distilled water - and no signs of overheating anymore. the manual says that going over 60% greatly impact the efficiency of the cooling systems - and it really seems to be the case.