Do you have a shower pump downstream of the cylinder, perhaps? If the pump siggnificantly lowers the pressure in the cylinder, the air bubble expands and air gets into the water stream, leaving less air in the cylinder. When the pump turns off, the air is compressed again but there is less now...
Before upgrading the pumps I would make sure the system is balanced. Even if the upgraded pump could push water through the more restrictive downstairs radiator circuit, it will still "prefer" the less restrictive circuit so if will make little to no difference if the system is not balanced...
I have a somewhat unusual challenge…
My underfloor heating system has been designed for a 60 degree flow temperature solely to satisfy the heat demand of a single double-height large room. In reality we have been running it at 50 degrees because that is enough except on the coldest days. It is...