Vaillant ecotec 466 boiler f.75 5er code?

Hi we have the above boiler which has been displaying the F.75 fault code. We have had the pressure sensor changed and the error is still happening. I have now watched the boiler as it goes into error and it goes like this:

Boiler heating up to 70C gets to temp and then there is a sound like a sigh. The temp on the screen then starts to climb and the radiator symbol starts to flash. The temp rises quickly to about 90C and then the boiler tries to reduce it - clicking sounds are heard and the thermostat control symbol comes on and off. The temp starts to drop as the boiler tries to correct itself. A code 5Er or SEr flashes on the screen. Then it gets back down to about 70C and the boiler then displays the F.75 code and stops heating the water and doing anything apart from the pump is still working inside.

The boiler is 3 years old and has been totally fault free until now. The people who installed have been back to try to rectify and have installed 2 new pressure sensors which have done nothing to help. The Siemens RDH10RF has been changed which had failed when the first sensor was replaced.

What else should be changed/ checked? I understand there is a diagnostic tool that can be used on these boilers too.

Please help.