how do i fix my alpha 280e boiler?

my alpha 280e boiler is not giving out on stop hot water it gets very hot then goes lukewarm the boiler fires up for about 30 secs then goes of then re-fires again for about the same time, Which i think is the reason why the hot water is stop starting all the time, the heating side of things seems to be fine rads are nice and hot. The problem is the same if the boiler is on hot water only and when it is on hot water and heating.ANY IDEAS!! MANY THANKS!