Thanks for the replies guys, much appreciated.
I have an inline filter, so dont think its muck in the pump.
When the engine is idling (nothing in use) it ticks over fine, with the release valve open and the water circling around the pump and back to the water tank. (i dont leave it idling for long, as i was told the water could overheat and cause damage to the pump?)
I then close the release valve and it can start to judder straight away, or sometimes when im using a fsc or lance (its all working fine), i then let go of the trigger, and the machine will judder around, when i then try to open the release valve, the valve will feel stuck and under pressure like there is air in the system?
Anyway, i will have a look at some of the things mentioned, see how i get on - many thanks