Sunfire Cooling System

I have a 98 pointiac sunfire 2.2L and i have a problem with the system getting back pressure and it pushes all the antifreez out of the system through the overflow resivior. what could be causing the back pressure? (i have changed the thermostat and i have also blead the system (which allows the car to work corectly for about a week and then it creates back pressure again).