That sounds like a hardware issue as described and when powering the wifi module it's effectively throwing a windows blue screen and rebooting. I'm sure there will be a diagnostic code you can enter into the phonepad to test wifi, if it reboots then you've got your answer, but service will know all that if you can't find a code.
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Sony Xperia Z Ultra C6833
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Samsung Galaxy Note GT-N7000, iPhone 3GS, HTC Touch Diamond, HTC Touch Pro, HTC Universal, Sony Ericsson P990i, boat load of nokia's for a long time.
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