One possibility is that the oven door is not closing properly and the heat is escaping through the gaps. If that is not it, check the heating element, thermostatnt, sensors.
The oven will need a new thermstat. Without this you will never get the right temperature while baking and will will eventuly not work at all. Going to an appliance store they will be able to help you find the kind that you need with the type of oven that you have.
If your lucky it is a problem with your knob. They can be adjusted on older stoves. You can also check to see if there is a bad or dirty connection to the internal thermostat.
It could be several things. Assuming that it hasn't been damaged and leaking, the most probable cause is a defective thermostat. This would be most likely if the condition has always been there. If it just developed recently, your electrical voltage may be low.
There could be a couple of reasons for this. The first that comes to mind is that the thermostat needs to be replaced. The other could be that the heating element in the oven is malfunctioning. Replacing the heating element isn't hard, and you can purchase it online.