Constant Computer Freezes When Using

I'm new to this forum, so please forgive me if I make a mistake.I recently purchased a Dell Latitude E4300 refurbished laptop from a seller on eBay. Ever since I brought it home, it has been constantly freezing randomly, whether it be whilst I'm browsing, navigating through windows etc.I've reformatted, and completely wiped the system about 3-4 times, installing different versions of Windows 7 whether it be Home Premium x32 or x64, or Windows 7 Professional x32 or x64 bit, but regardless of what I do, it concludes pointless. I first suspected it was the graphics driver, hence I've tried updating, and downgrading the driver to see whether it would rectify the problem. After that appeared pointless, I suspected it might've been the fan, as the computer does heat up quite quickly, however that concluded pointless as the computer froze even on the windows 7 boot logo. Finally I suspected it might've been the hard drive, and/or the ram, I did some tests (Windows Memory Diagnostic & Memtest86+) and CHKDSK multiple times without any detected errors being found. What I found interesting was that it mainly froze when I initiated a "Windows Experience Index" test, however that was at the beginning.