potterton combi 80?

hello, my boiler has always had a slow hot w outlet that, once it has been on for a while is scalding hot.
I increased the cold w inlet thinking this would both improve the flow and cool it down which it did to a certain extent.
After a week or so the boiler seems to have developed a fault, the flame does not go out as it used to when the hot w outlet is turned off, it runs on for a minute or so making hising\ fizzing noises....eventually it goes out or the lockout is activated. opening a cold water tap elsewhere in the house seems to stop it.
I have turned the cold w back to its original flow but this has made no difference......