Oven does not always heat to set temp

My oven seems to have a mind of its own. Sometimes it will heat up to the correct temp and other times it doesn't. I always know when it's messing up because it will stay in preheat for a long time and still not be at the right temp. The broiler works fine. All of the oven controls are digital push button, so maybe there is some sort of short in the control panel?
I'm about ready to call the repair man! But thought I would try here first. If it's possible to fix this myself and save some money, I'm all for that!

Thanks in advance for any help you can give me.