I have a relative new and expensive oven and never thought that I needed to check the temp. (I am a hobby baker.)
Yesterday I was buying supplies and came across an oven Thermometer and thought, why not.
I can't belive my eyes, 
I started at 325 and it did not read near the 300 mark, I have to set my oven to 380 before the guage read 325. I would never have thought. My cakes seamed to be baking at the normal time + a few.
If it is under warranty they will come and fix it. The repairman showed me how to adjust the display to the right temperature. Mine was off 10 degrees at every temp, so we just changed the display to match the true temp. If it is not holding the temp, that is definitely a repair job. All ovens come on and off to maintain temp, from 10 to as much as 25 degrees as the variance. Most expensive ovens have very good customer support online and by phone. It is so frustrating to get ready to bake and the temp fluctuates.
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