C6603 Overheat and shuts down

I have had a problem recently when i turn on my phone the phone after a few mins will start to shut down and say its due to overheating. I have done repair with sony companion but still nothing i thought i would downgrade to andriod 4.4 still nothing and then i downgraded to jelly bean. still the same problems.
would it help if i put a custom rom and fix the clock speed ???

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