One of three things are happening. You should probably take care of all three at the same time.
First - Thermostat and Gasket - You should replace the thermostat and gasket. It is fairly inexpensive. If the thermostat opens to soon, you wont get very hot air inside.
Second - Clean the area between the Radiator and Condensor and verify fan operation- This is probably what is causing your summer issues. Make sure your cooling fans (radiator fans) are turning on when the temperature gets higher than "midway" and turns on when you turn on the a/c. Also, the area between your radiator and condensor MUST be completely clean from any leaves and/or cotton from trees. You can blow this area out with compressed air or with a water hose.
Third - Lastly, flush your cooling system. This should take care of any buildup in your heater core. Heater cores blocking up is not very common in this car. They usually are good until the spring a leak which you will notice by your windows fogging up and not going away.
Seems like you have 2 issues, summer issue which is probably First or Second Repair and the winter issue is probably First or Third Repair.
All three of these repairs are fairly cheap and doesn't come close to the price of a new Heater Core.
Good luck and let us know if you have any other questions.