Ge double oven - both ovens stopped heating gradually

My first thought is the power supply to the unit.  Cut the breakers and inspect all connections from the fuse box to the oven.  You will likely find open connections or corrosion.  One breaker could also be bad.  Check the voltage with appropriate caution and a voltmeter.  Breakers should respond with a snap when turned on.  
Note: there are two breakers which control the oven: one with a red wire, the other is black.  Each provides 110 volts to the oven in combination with the white neutral.  Together, the black and red give you 220.  If one of the circuits is out or in poor condition, the bulbs might not work and the oven won't either.