Oven gets to temp, but won't stay there, unless a top burner is on too.

JGB920SEC3SS
Hi there,

GE XL44 Spectra 61/2 years old. Oven not staying up to the set temp. It may reach the set temp just fine but then it wont come back on by itself to keep the temp. Often we have to turn on a top burner to get the oven to "start" the temp rise.If we leave on one of the top burners, then the oven will stay on the set temp.

I have seen numerous “oven not keeping the set temp” posts. But I have yet to see one that mentions the top burner being on to keep the oven at the set temp. I understand it might either be the igniter (glow bar?), the safety gas valve, or the control board. I was guessing the gas valve, but then after reading here, maybe the control board?
Is there any diagnostic that I should try to eliminate one or the other? What should I try to replace first?
Many thanks

Any thought where I should start? thanks,Mike