sime boiler super format 100 keeps cutting out/please help?

Hi,
I have a sime boiler super format 100 combi.
ive put the heating on and after about 30mins it goes off,but the boiler is still running but not firing.I have waited for the burners to come back on but nothing so i have to turn the CH off then turn back on and it will keep on doing this.

i had a plumber come out but he could not work it out,its been happening for 2 weeks now.
please can someone help...
water pressure 1.5