Clearly your drive has 4 heads. AFAICT the performamce dips are spread randomly over all 4 heads. One possible explanation could be a background Windows task which is interfering with the test. An alternative explanation might be oxidisation of the HDA contacts on the PCB. This is where the PCB mates with the headstack connector on the drive. You can clean these PCB contacts with a soft pencil erasr.
See http://malthus.zapto.org/viewtopic.php?f=86&t=649&p=1789
These are the HDA contacts:
http://malthus.zapto.org/download/file.php?id=420&mode=view
