I have a feeling that the ABV when opening is allowing trapped air in your system into the return and causing all sorts of issues (not the ABVs fault) , if a AAV or manual air release isn't fitted then how was the air in the system initially released on refilling the system I wonder, the boiler AAV is fitted to remove air from the boiler and not the system but may do so over a prolonged period but I wouldn't bet on it.
One other query, if there is a pressure gauge in the boiler display, it should read ~ 1bar when cold and ~ 1.5 to 2.o bar hot.