2002 ford focus overheats and doesn't mark it on inside is always in the middle.?

Replace thermostat, timing belt, relay, water pump, cooling fan sensor, and also checked for blown head gasket that was ok. But whenever is on for a while the resevoir tank starts to boil and throws the water out. Fans turn on when a/c is on but not when a/c is off. What else could it be?Need your help!
 
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