We appear to be having the same issue with our Optiplex 790's that are configured with the Seagate ST250LT0007-9ZV14C drives. We have several hundred 790's deployed with Toshiba and Western Digital drives that aren't having any issues. However, the units with the Seagate ST250LT0007-9ZV14C drives are behaving just as you described. Typically, they run fine for 3 to 6 days and then the Disk Response Time suddenly becomes extremely slow. As you mentioned, rebooting does not help, we need to shutdown the PC to resolve the issue. We're running Windows 7 64-bit on our systems instead of windows XP and we're not using RAID. I also have a case opened with Dell Tech Support.