Hey Dave, It's possible the thermostat has slipped out of calibration. It may be able to be adjusted to work better, but doubt it will adjust enough to be normal. Is this a model with separate function (Off, Bake, Broil) and temp switchs? If so, GE may still stock a thermostat that will work with this range.
If the range is a old as you say, it may have coil oven elements instead of Calrod - not sure when they switched over. Are all of the elements heating as they should when it's on? If they do, and they just shut off too soon, then it most likely is a problem with the thermostat. If some of the elements don't heat at all, they may be burned out or a wire may have come loose, or the contacts in the control may have gone bad. I would have to see a wiring diagram for this particular model to know how the elements are connected and switched. Hope they can get it working without too much trouble.