Cooling problem

Hello I have a problem with 97 lumina cooling system. so a few weeks ago it overheated so i decided to change the thermostat and everything was fine. now all the car wants to do is shoot all the coolant out the overflowtank, as if its not even going through the system. i checked the water pump and changed the rad. cap. but still it is doing the problem. a mechanic flushed the radiator out for me but that still didnt fix it. also it seemed to be fine for a few days after i changed the thermostat and also after he flushed radiator but then two days later it does it again... just wants to keep shooting everything out the overflow. the mechanic says he thinks the fans might not be coming on in time, so right now i am going to try to jump them to keep them on all the time and see what happens. but i was hoping for any other things to try to help me fix the problem for more than two days lol... thanks to all that respond.

If it overheated before, and you replaced the the thermostat, sure the system was properly bled? Does the gage have wild swings and overheat? Do you lose hot air from the heater core?

Those fans should come on between 210-220F.....does your A/C work? If yes, do the fans run when the A/C is turned on?

It's possible you may have a head gasket problem......do you know how to check for that?

+1 on the improperly bled. If it will bleed out and run ok for a while then starts doing it again, with air appearing at the bleeders and rejecting coolant to the reservoir, you've most likely got a compression to cooling jacket leak.