how can i sort this?

my sime super four combi keeps cuttig out and the pump is running.the pressure is up to 3 bars and ive tried to drop it by lettig water from radiators.it will not drop below 2.i have released the pressure with the red valve at the top and still it builds up again.i then turned the mains supply off and the pressure stayed down. i emptied the air in the red tank which was water free and repumped to 1 bar. turned mains on and the sytem fills itself and runs at just over 3 bar