unusual sequential read graph on CF SSD

hi,
i just got a 2x4Gb compact flash drive set up in Raid 0 with a CF/IDE
raid PCI card (http://www.addonics.com/products/flash_memory_reader/
ad4cfprj.asp).
the HD tach sequential read results start at 28MB/s and slowly rise to
50 and then slowly fall to 28 again. I don't understand why the read
performance should vary so much. can anyone help me understand why
this is? i have 64k chunk sizes on the stripe set. Theoretically each
disk can read at 40MB/s but it's fake raid so i'm not expecting to
reach that. the graph is available at http://tim.mackey.ie/dev/CF_SSD.jpg

thanks a lot
tim