ideal pressure over heat problem?

Help !! like alot of you out there i have had continued problems with my ideal response 120 every now and again without warning when i run the hot water tap the boiler makes an odd noise the pressure rises rapidly and it spits the water out the overflow and cuts out. i have replaced the flow switch previuosly and also a curcuit board which got the thing working but now here we go again more ££. does any one have any ideas ? any advice would be much appreciated.