central heating?

i have a icon28t boiler & whether i turn on the hot tap or switch on the heating,the boiler switches on &runs for 10 seconds & then turns off but keeps on repeating switching on,runs for 10 seconds &switches off.sometimes you can switch on the boiler& it will run perfect for 30 minutes& then when it has reached its temperature &swithes off,,when it trys to ignite again ,,it runs for 10 seconds& then switches off. can anyone tell me which part is faulty