Potterton Prima F cycling?

I have noticed over the last few weeks that my Potterton boiler is cycling much more frequently than usual. When it get's near the desired temperature it tends to start up for approx 30 -90 seconds then shuts down for a similar period until it gets to the temp. I have had British Gas out 6 times where they have replaced the pump, room thermostat, boiler thermostat, PCB and some other componenets but I am not sure what they were. I mentioned that I had put Fernox F5 through the system and they have said that this would cause the sytem to work much more efficiently and therefore the cycling will occur at quicker intervals, I am not convinced and the boiler is still switching on & off frequently. Any ideas?