|Really never easy to find sometimes.|
The problem with diag tools is they don't replicate what you did with the actual OS and applications.
Start at bios maybe. Set to default or fail safe settings if you write down the other settings.
Might be time to re-seat the ram, could be a bad gate somewhere failing, And there are a LOT of gates in a computer.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10