I had the same issue and then concluded that time machine is not stable over the network. The use of the sparsebundle is basically a way of pretending that the network drive is actually a local disk and it will corrupt itself on network faults. You can probably make it work semi-reliably over a wired network but forget wifi. This is a failure of Apple's design and there's not much that can be done until they implement a sane network aware solution.
