I am having the exact same issue with 3 optiplex 990 clients purchased from Dell, all using Seagate ST250LT0007-9ZV14C drives in a RAID 1 array. I reformatted all of the systems immediately after receiving the computers, installed Windows 7 x64, DSS, dell drivers and applications. I can verify that reformatting the HDD will probably not fix this issue. All 3 of the clients are doing the exact same thing, work great for about a week, then HDD transactions slow to a virtual crawl until the system is powered down. I've read other cases where the customer has forced Dell to replace all of the drives with a different model alltogether, and I'm about to see if I can get them to do the same thing for us. Good luck, and I hope this helps.