Have you tried running in 32 bit and seeing if the problem persists? If it goes away in 32, that in combo with the Plugin Manager issue might mean something.
I’d first try running in 32 bit, and if that doesn’t make a diff, pulling all of your 3rd party VST plugins out of your library and troubleshooting from there. It’s a chicken dance, but… if you don’t have a hardware issue, this shouldn’t be happening!
Also… is your Quad PCI or Satellite?
Chewy