whats wrong with my worcester bosch greenstar 25si?

Hello,iv got a worc/bosch 25si/30si combi,which has been switching itself off.When i attempt to switch it on i get a lot of clicking and whirring sounds but fails to switch on.Eventually it will go on for a while and radiators heat up but(sometimes)not as much as they should,when its working the pressure gauge shows just at the top of the lower red segment.Any advice would be much appreciated.The boiler is 4yrs old.Thankyou. Craig.