18-month old Gas oven, Kitchenaid KFGG500ESS0, oven won't maintain temperature. Heats up once, but then coasts down. If you turn it off and back on sometimes it will re-engage and heat up again, other times it won't. I've replaced the gas valve in the back and it worked perfectly for about 2 months, now it's not working again. I now suspect the temperature probe or thermal switch are malfunctioning. Since I'm basically shooting at it with the parts gun I'd prefer to start with the cheap ammo. Any suggestions? I don't want to start with the control board unless the forum thinks that's the likely culprit.

Thanks,
-Dave