The main burner on my Potterton Prima C keeps going on and off?

I have a Potterton Prima C combi boiler, the main burner keeps going on and off. It comes on for about 5 seconds then goes off for about 30 seconds and so on.
We have had the thermostat replaced last year as we were told this was the cause (this did not recitify the problem), we have just had another gas engineer out and he said he thinks it might be the thermostat (didn't work last time)but never came back to replace it. Does anyone really know what is wrong with my boiler. Thanks