when I upgraded from Fedora 12 to Fedora 14, about twenty days ago, the system (which wasn't doing really well even before the upgrade) became almost unusable.
Weird problems often have a hardware issue behind them. There are many things to check. Some are easier than others:
From the desktop, open the program System>Administration>Disk Utility.
Select each of your hard drives in the list on the left, and then click the Smart Data button.
In the upper part of the window, look for a Bad Sector count, and Overall Assessment of the drive.
Any problems?