Do you sometimes smell gas on the first light up?
The ignitors last for about 5 years on your style of range. They will start taking longer to ignite. It should light in under a minute. If it takes longer than that it will be the ignitor. Also if you have the bottom of the unit out and watch it ignite the flame should come on full size right away if it starts with a small flame and gets larger after a nother minute it is a bad ignitor. If you think it is the computer or the sensor you will have to do some electrical measurements to see wich one is failing.
It will most likely be the ignitor. If you want you can also get it locally by using part #WB13k4. If you think it is the control or the sensor you may want to have a tech out to look at it. You do not want to guess at the control it is not cheep.
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Some of these units you can switch the broil ignitor to test it if you do make surre to unplug the unit.
Most of the time the electronic temp control will be bad through out the temp range not just at 350 or 400. Make sure you do not use foil in the oven. Shinny new pans will also reflect heat and cause a longer cook time. Some glass pans will also take longer. If your thinking the oven is holding temp. get a cheep white cake mix. (about a $1.50) pre heat the oven for 30 minutes. Follow the mixing directions exactly and place on the correct rack. It should come out perfect with in two minutes of the mean time given on the box. If you have items that are going to be in the oven for more than an 1.5 hours bump the temp up 15 deg. The long cook items do no do as well with the tight temp control of todays ranges.