Frigidaire Oven Preheats, but doesn't keep heat

It turns on and preheats. Then preheat light turns off after it reaches temperature . But the set temp doesn’t keep. So food takes longer to cook. Trying to figure out why it won’t keep the heat at temp even though it did preheat.

Could it be the thermostat? I don’t think it’s the igniter because it does ignite.

Thank you in advance for the help!

If its gas and has a glow bar igniter, the igniter could be starting to fail. They work when cold, but become intermittent when hot.