Readiing a previous question regarding determining the accuracy of oven temperature I wanted to comment/ask more on the topic.  My GE Profile wall oven is 17 years old, its electronic board was replaced a few years back and all seemed fine until recently.  I have two oven thermometers which give varied results as much as 50 degrees off.  I have noticed that my oven does seem to get more accurate at higher temperatures.  Set at 350 I know it is less than 300, set at 390 I seem to get close to 350, set at 475 for pizza baking on a stone (which is always in the oven) and the crust bakes just fine.  Has anyone experienced a "sliding scale" for accuracy?  Anything I can do? 
While I am unsure about the 'sliding scale' you mention I do know that there is something you can do about it. A while back I discovered you can calibrate your thermostat. I couldn't find (quickly anyway) a good video example for the GE Profile, but did find two good GE examples.   For an analog (dial) type thermostat see this video.  For a digital thermostat check this one.   Specifically I would suggest finding your particular owners manual (most of them are available online as a PDF for download anymore)  As @moscafj points out if you are using two thermometers which are not in agreement you may want to acquire a new one and treat it as "right" for the purpose of calibrating the rest (or figure out which one is the odd one out and throw it away...)   Some additional notes:   Your pizza stone is likely acting as a heat sink (as it should) making it's behavior more consistent than if you were not using one, this would tend to 'explain away' the sliding scale theory. You need to be conscience of how often you are opening your oven door. This will have the effect of allowing far more heat escape than most people are aware of. If you are frequently opening the door to check the temperature you are possibly doing more harm than you know. This, too, could explain your observed sliding scale, at least in part.  