my oven lights but does not come up to the full flame?

Have a sroves gas oven and when i hold in the control knob and turn it round to full, the small flame lights up, but it is intermittant as to whether the full flame comes on after a minute or any asmount of time, so that i can then seelct the appropriate cooking gas mark. When i say intermittant, i mean that i can someitmes get it to work by waititng a while with the small flame, then after 10 mins, trying again, or sometimes by once it has been part lit for short while, pushing in the control knob again, or even sometimes by oipening and closing the oven door again.
any ideas which part is causing the problem would be greatly appreciated.
thank you