Well, I think I spoke too soon. It's doing the same thing again... Adding more turns on the internal bypass did not help - put it back to the original position now. Does this boiler not have a flow rate sensor for the heating circuit? I'm sure the flow rate is good, because I have 3 towel rails all fully open and they heat up very quickly when the boiler outputs 60C.It's still a strange one though because if the diverter valve was working originally its hard to understand why not enough water flowed even if the bypass was fully closed, the diverter valve should have directed the primary water to the zone(s) valves which presumably opened at the same time and even one rad/towelrad should provide the minimum flow necessary.
So the situation now is that it would sit at minimum flame for a while with return temperature about 38 and flow about 42... If I turn the hot water it heats it up perfectly fine - flow goes above 60 immediately... Then after DHW pump overrun it goes back to heating mode and temperature drop to 40 again. What else could be wrong?