My Mac Mini 2011 i5 Instant Shut down while using high CPU?

So ive had my Mac Mini for 5-6 months and it just recently keeps instantly powering off after using high CPU, i think it something to do with the power it needs, i tested this thoery by using 'GeekBench' and 6 seconds into the text which pushing your system to the limits it power out quite agressively, i think its a power issue because when it shut off unexpectedly, then when you go to turn it on again it wont, unless you plug the power cord out of the mac min and back in again??

Anyone with any thoeries im ussualy good at solving problems but im stumped on this :S.... as you can see it runs lovely untill you push it to even half its power (CPU wise = power needed)

Specs :

Model Name: Mac mini

Model Identifier: Macmini5,2

Processor Name: Intel Core i5

Processor Speed: 2.5 GHz

Number of Processors: 1

Total Number of Cores: 2

L2 Cache (per Core): 256 KB

L3 Cache: 3 MB

Memory: 4 GB

Boot ROM Version: MM51.0077.B10

SMC Version (system): 1.75f0

Software :

System Version: OS X 10.8.2 (12C60)

Kernel Version: Darwin 12.2.0

Boot Volume: Macintosh HD

Boot Mode: Normal

Computer Name: andrew’s Mac mini

User Name: andrew simpson (andrewsimpson)

Secure Virtual Memory: Enabled