how can i fix my vokera options boiler it heats up and cuts out after ten s?

my vokera options boiler heats up then cuts out after about 10 seconds ,the flow pipe at the boiler gets very warm but my rads are not very hot ,have tried flushing system but that had no effect ,pump makes a humming sound when boiler does fire and the flow and return hoses at the heat exchanger seem very hard and brittle. When using hot water,the water runs hot and cold.I was wondering if the hoses were blocked thus restricting the flow of water through the boiler also since i flushed the system it sounds as if there is grit in the impeller housing