Soldato
If it was me and this was a newly bought PC and the site that I bought it from could not find the fault remotely (IE. by talking you through things, or getting you to return a specific part etc.). Then I would speak to them again and politely suggest that the PC was not fit for the purpose that it was bought for and request that they uplift the entire PC at their expense. Only my opinion of course. Obviously you need to give them a fair chance to sort this. But ultimately, if it does not work to your satisfaction, then it's their responsibility. They are a pretty fair company, so I would have thought there should be no problems.
Good luck.
Good luck.