Your post is light on details but there are a few issues with the specs. 12GB would be rough to do since there are no 6GB memory sticks. To sniff out possible RAM issues you must install all the RAM in matched pairs or just a single stick. At the end will be a link so you can easily share PC details.

The PSU is unknown. It could one of those iffy multiple rail models. Tell the forum the exact make and model or supply a link so folk can judge if this is a possible problem unit.

Post the Web Speccy report for more. How is at https://www.piriform.com/docs/speccy/using-speccy/publishing-a-speccy-profile-to-the-web