I had a similar problem on my machine

After using standby (accidentally as I had left the PC for a while and had forgot to change power management settings) the whole OS was very laggy, System monitor showed high CPU usage.

I think the OS crashed when trying to resume from standby and after a reboot then I had the high CPU usage problem. No matter how many times I rebooted, nothing fixed it.

I only have a single core Athlon 64 though. Also, booting with -x option would make it run smooth but video drivers didn't load so couldn't run anything fancy.

Never worked it out and eventually reinstalled