Could the PSU be failing under load, even though voltages tested ok? Or, is PSU overheating? Does its fan work?  Have you run Dell's BIST test? Did thermal paste between CPU and heat sink dry out and separate?  If it's not the PSU, it might be worthwhile removing the heat sink, gently cleaning both surfaces, applying thin, fresh coat of decent thermal paste, and reattaching heat sink.
My Inspiron 5675 crashes while gaming, any game whether it's Steam, Battle.net, Ubisoft. It even crashed once while editing an Excel spreadsheet, but mainly during gaming. I can't see any heat issues while running HWInfo and there are no events in the viewer leading up to the crash that explain it, just notifications of an unexplained system shutdown. I'm guessing a heat related issue that I can't monitor? I followed the steps mentioned on the Inspiron board re: USB and power Shutdown settings to no avail. But it's not a total shutdown as there remain lights on the PC so the power supply is still supplying power, just the computer goes dark and requires a hard reset to get it running again. I tested the PS with the standard AT tester and all voltages are green lit.  I had previously installed a Hynix SSD on a PCI riser that has functioned well for years.  I replaced it with a new Hynix SSD and placed it on the motherboard (removed the PCI riser) but still get the random shutdowns. I don't see any online talk of Hynix being prone to overheating but I also don't have any heat plate on either the riser or the motherboard (never came with one) but see talk online of them and wonder if I should have had one or need to get one? How could I tell if it's the SSD that's causing the shutdowns? I've scanned the OS for corruption and the disk for errors, and all turns up well.  Thanks for your interest.   
Inspiron 5675 overheats