How can i fix my potterton combi 80?

My problem is when i switch the water on it takes ages for the flame to ignite, once it ignites it only stays on for about 30 seconds so i have no hot water, my second problem is the central heating flame comes on but only stays on for a few minutes, on my thermostat it states that the heating is on because it shows me the flame sign but the actual boiler is not running