New memory fitted, now turning off randomly

I have just upgraded my 27" iMac to the full 16GB RAM. It recognises it fine, however the computer has started rebooting randomly. No warning, it just turns off, and then bizarrely there's no error message when it restarts (in my experience the computer usually knows when it has crashed and offers you the chance to send an error report.)

Here is the exact memory I bought:

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItemVersion&item=260814559124

It was cheap, but does claim to be good quality. Can anyone give me any ideas as to why the computer just turns off? Are there any error logs I can check?

 

Thanks for the reply. Is there any way of actually testing which module is bad? Or seeing if the iMac considers it compatible? I want to have some idea of what I'm talking about before going back to the seller and complaining.