car engine overheated

I have a 2000 buick Lesabre v-6 a while back the car overheated and I believe it was a quart low on oil. after it cooled down and we added water and oil we were able to drive it home we put it on a check machine at auto zone and it said it was a problem with the number 4 plug or wire not firing. so we changed all the plugs and wires. then we put the blue devil in the radiator. it has run somewhat hot but not as bad since but for some reason we have to take the radiator cap off before we go anywhere and let the water from the overflow drain down into the radiator after we do that the overflow is still on the fill line so were not losing and water it just isnt cooling the engine and running thru on its own what could we do to fix this problem.