Combi boiler - Return temperature going up

[Renato]

Hi,

The outlet of all my radiators are at 55°C, but the boiler keep showing the Return Temperature is 60°C (and keep raising at 1°C/min rate)

Also the flow temperature keeps going up above the set point and when it reaches +5°C above the set point, the boiler shuts down (even before the room reaches the temperature).

After 3 minutes, the boiler starts again, but because the flow temperature is already high, the temperature reaches 80°C in 10 seconds, and then the boiler shuts down (for safety reasons).

Could someone have an idea what could be wrong?

Boiler: Potterton Promax Ultra Combi 40 erp

Thanks,
Ren