My Gas Slide-in Range (GE Model JGSP20GER2BG Serial# AH259094Q) has been giving trouble for some time. The oven has taken longer and longer to bake. I purchased a separate hanging thermostat because I suspected that the oven thermostat may not be working. I have now confirmed that it is not working properly. I have the oven thermostat adjusted to +35 degrees hotter and I still have to set the thermostat to 430 degrees to achieve a 300 degree oven temperature. Does the oven thermostat need to be replaced? I installed the range in 1993 and ithe oven worked fine until about a year ago when it seemed to start drifting. I am capable of doing appliance repair if I have a repair manual or clear instructions. Any suggestions? Do I need to replace the oven thermostat? The oven is only used for baking bread, pastries and the like (no meat or other greasy items).