in response to Shticky Guy, I wanted to share two comments.
1) Regarding a modern computer-controlled oven with a Tweaker, Rav Belsky, Shlita, and others permit adjusting the effective temperature of the oven both up and down, using the technique of extending the present phase of cycling. The Torah Technologies Org web site has his ksav hechsher for the Tweaker and more information.
2) Regarding an old style “landlord oven”, Rav Blumenkrantz zt”l in his Pesach Guide quotes Rav Moshe Feinstein zt”l saying “one may shut the gas (for some good reason) on Yom Tov” – please see page 12-438 of the 2004 Pesach guie. He also says for an oven with a gas pilot: “when the flame is on, one may adjust the flame on Yom Tov as deemed necessary.”