Question PC cccasionally shuts off, do I need a higher wattage PSU ?

Can you narrow down when the system reboots on it's own? Keep an eye on temps. What are your ambient room air temps? BIOS version for your motherboard at this moment of time? Check and see if any items in Device Manager are flagged with a yellow exclamation mark.
 
Using another person's overclock figures without careful stress testing your own system can lead to instability. No two systems are exactly the same. What works on one system may fail in another. By all means use their figures as a starting point, but do your own thorough research afterwards.

I actually under-run my GPU using MSI Afterburner and set the power limit to 95%. It was a simple fix for a video app still under development, which caused random computer crashes part way through a 10 hour render session.

Sometimes stability is preferable to speed.
 
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