how can i fix my vaillant turbomax plus 828e combi boiler (VUW GB 282/2-5)?

The central heating working fine but the boiler supplies only Luke worm water and after every few minutes the gas burner goes on and off and the water temperature falls to zero,recently changed the divert er valve but still remain same,the engineer suggested that the boiler should be remove from small kitchen to other suitable location which will be costly,i will be grateful for yours kind help and
suggestions.