How can I fix my Glow Worm 30e boiler?

My boiler is only only coming on in short intervals to warm the radiators, however the water is still warm whenever required. It switches it self on when selecting radiators for a short time then powers off after a relative short time. Therefore, the radiators don't have time to warm up. We have had the boiler for approx 8 years and had it serviced 2 years ago. It needed a diverter valve to be replaced, and we have not had any other issues until now.