Sable overheats

My 1998 mercury sable quit heating last winter so I had it flushed twice and backflushed once. It never heated next step was to take all the hoses off and flush them and change the thermostat helped for about a week. Started overheating so I looked into it further and changed the overflow tank cap. Run a couple weeks w/o overheating. Overheats again ( my fluid always comes out of the cap on the overflow tank. I found out the radiator was packed full of rust so in turn I changed the resevoir tank, radiator, thermostat(again), and the water pump. Car worked great! Never overheated and the heater was kicking out the hot air For A MONTH now the guage inside the car never gets to the hot mark but fluid is coming out of the resevoir tank again. There is no actual radiator cap on a 98 sable and along with this overheating problem my blower fan works sometimes and sometimes it does not. Can anyone help me with this problem becuz' I am about to go insane with this car! It should be fixed with all the new parts but still it is not.