Our 8531 had problems like that and it turned out to be the burner thing--a big tongue shaped metal part with the sparkers out over the top of a screen and the gas comes out under the screen and the flame is there. On ours, the screen was all rusted out and this distorted where the flame was supposed to be so the sparkers were not getting a proper reading. I managed to find a replacement burner and all is good.
There are a couple of youtubes showing how to get at the burner. Hardest part is putting the screws way in there back in--you need a magnetic screwdriver. Also hard part is shoving the inlet gas tube out of the way so you can get the big piece out that has the burner stuff attached behind it.
