GE XL44 Gas Range Temperature Drift...Inconsistent Temperature

I have a 3 1/2 year old GE XL44 gas range that will not hold a consistent temperature. For instance, if you set the dial to 400 degrees it will reach 400 but over the cooking period will drift back to say 350 degrees and back up to 400. It is not consistent. My wife bakes a lot and first noticed it when some of her baked goods were not getting done. Any ideas?

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