So, for years we used the "preheat" cycle on our oven. 4 minutes and we can start cooking things? Awesome! Or so we thought.
One day we bought an oven thermometer, mostly so I could work on my barbecuing method (smoked-meat barbecue, not charcoal grill barbecue). Lo and behold, the temperature was 220 after the preheat cycle was finished. Gee, I think I know why we felt our oven was temperamental and not all that good.
Ignore the preheat cycle - it just warms the air inside up, and not all that well.
For 350 degrees, let it heat up for 15 minutes.
For 400 degrees, let it heat up for 20 minutes.
This will instantly make your oven MUCH better than it ever was before.