Frigidaire Oven won't maintain heat

Oven Repair (including Ranges and Cooktops)
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My oven (FGF326ASE) will heat up but won’t hold the set temperature. I have checked the oven sensor and it seems to be reading the correct number of ohms. It occasionally will work (usually when I am testing it), but often won’t (usually when we are trying to cook something). Any thoughts?

Thanks.

Steve