Update:
-MemTest came back with no errors and no crashes.
-Prime95 processor stress test came back with no induced crashes and no crazy temps.
-FurMark GPU stress test came back with no induced crashes and no crazy temps.
-OCCT CPU/GPU/PSU stress test came back with no induced crashes, crazy temperatures, or voltages way out of spec.
-After completing the tests, TF2 ran just fine for over an hour with no stupidity.
-Shut down system, opened case, no leaking capacitors, everything is seated just fine, no damage to PSU connections, blew out minimal dust with a can of compressed air.
-Booted system, checked all temps and voltages... everything was normal.
-Ran TF2, joined a server, connection timed out, sat in server browser trying to refresh (it wouldn't), and the crash happened.
-Event log is still blank except for noting an unexpected system restart (this timestamped after the crash), and unfortunately I did not have monitoring software active in the background this time.
New hypothesis (input appreciated!): Crashes are cause by a conflict with or dying onboard network card. Evidence: Every single crash has occurred during [intensive] internet activity (TF2, downloads), several programs have been timing out or dropping connections to remote servers (Steam, Mumble, TF2 - this started happening a month or two ago and hasn't gotten better with an ISP plan upgrade), and no games have caused the crash other than TF2 (which is currently my only multiplayer game).
An update was deployed for my network card through Windows update last night as optional, which I installed following the last crash. I plan to try things further today.
Anyone able to comment?