Halstead finest gold safety lockout problem?

Halstead finest gold boiler has started to show solid red safety lockout when hot water is turned on, This does not happen when I try central heating as everything is fine? It started intermittently but has steadily worsened. I cleaned the venturi as I usually do around this time of year in preparation for winter. I also replaced the air pressure switch for a new one as I had been told that 80%of the time it wil be this causing it, But it still does the same. Some times everything fires up great and we get hot water then the next time it goes straight to red light, not flashing but solid? Any ideas please would be very helpful