Gas stove will not heat to proper temp

I have been having the problem of my gas oven not heating up to the proper tempature since january and it seems to be getting worse. I placed a thermometer in the oven to see just how hot it was getting compared to the digital readout. The readout can be 50 degrees or more off. It is taking twice as long to cook anything in the oven. The stove top works just fine.

You can adjust the temperature as described in the Owners manual (page 9).

Gene.