Eno cooker problem

Hello all,
I have an Oceanis 361 with an ENO 0420 cooker. The problem is:
After the large burner has been alight for a few minutes, when I turn it down to low, and then up again, the flame ignites under the burner cap and burns very yellow with lots of soot on the pan.
Has anyone had a similar problem and found a solution? (everything is clean no blocked holes etc.)
Any help gratefully received.