how can i fix my potterton oil fired flowsure plus boiler?

hello my boiler is potterton flowsure plus,it is oil fired.The boil seems to work fine when centrl heating is on,but when we run hot tape or shower it is o.k for a few mintes and then the water cools off slowly.The boiler fires up soon as the tape is run.the boiler is up stairs ,and about 6 years old,it also is serviced about evey 6 to 8 months