For mysterious undiagnosable seemingly hardware related issues, the power supply is always a good place to start.
Be aware that Dell has a nasty habit of using "proprietary" power supplies that have the pins switched around on the ATX connector, and thus won't work unless you either buy one of their stupid OEM supplies or get a pinout chart and rearrange the pins on your new supply before you plug it in. I don't know if that model is one of the ones that does this, so you might want to check first before you potentially smoke your board.
Every once in a while my machine requires cleaning, not just reseating, the RAM and/or video card edge connectors and slots for no readily identifiable reason. I have no idea how any kind of crud or oxidation manages to accumulate in there given that my PC never moves, it's not in an especially humid environment nor one with temperature fluctuations, and as far as I can tell the air in here is acceptably clean. But it does nevertheless, and when it gets into that mode it will randomly reboot or blue screen with no rhyme or reason until I remember to hose everything down with CRC contact cleaner. I have to do this once about every 2-3 years. (Yes, I have had this build long enough for this to happen more than once...)