Engine overheating due to contamination of coolant

What is your question?

engine overheat due to contamination of coolant and engine oil mistakenly poured into the radiator reservoir, total cleaning of radiator & reservoir tank were completed and changing of thermostat were already done and even cylinder gasket was being replace but still the engine overheat exist, what would be other problems that keeps the engine overheating, hope to get your advise and answer to my question.

What makes this problem better or worse?

few minutes of engine running the water inside reservoir tank starts to boiled

How long has your 2012 Ford Fiesta had this problem?

few months already