Ok I am able to narrow the problem better now.
Whenever I leave my Dopod 900 synced connected with my laptop and when there is a phone call coming in then I proceed to unplug the usb to pick up the phone, This may result in some sort of data corruption in syncing status, I think. Because after that, when I try to re-synhronize the dopod 900 again, it would try to synchronize from ground 0, so it will delete every synchronized data such as contacts, tasks, calendar in my Dopod 900 and re-transfer from desktop computer into this device all over again.
This is very scary and unacceptable. Could anybody else replicate this problem?