For some time now, the flame on the top of the cooker is weak. The 2 small rings are quite good, but the large rings are feeble, with yellow and blue in the flames. When I put those rings on with a pan on top, the flame will start sputtering, on and off and make quite a noise. I've cleaned what I can get to, including lifting off the individual 'burner plate?' and the small metallic ring, to get to the gas hole which I cleaned with a toothpick. But it's no better. It's a great solid cooker, so I'm hoping you can help. Thanks.

beatrice straker, February 2017