This is what I got yesterday:
These are the results from today:
All I did was install Windows 8 on it. But after that I formatted the drive, later I even ran the clean command from diskpart, so I don't think that should have anything to do with it.
Also as you can see from HDTune it's the same jumpy performance all along the space on the drive.
Intel 520 180GB, Gigabyte P55A-UD4, using the "native" SATA-2 controller, not Marvell.