1pm-5pm? As for the fluctuation in stock is probably canceled orders or amazon also receive shipments to its 4 or more warehouses which you could see some lag in the numbers they are able to provide.
http://www.eproductwars.com/vg/asinsaleshistory.cfm?asin=B0009VXAM0&db=vg
Stock tracing history going back 20 days. Can't really see the past 3 days but there has been alot of activity.