Time Machine over the Network

Hello,

I've been using Time Machine for over 3 years now with no issues over the network; the destination being a 1TB-limited sparseimage setup as instructed by this tutorial. The image resides on an external HD connected on a network-connected iMac. Everything has been working flawlessly: Every hour Time Machine would MOUNT the disk image on its own, BACKUP the data and UNMOUNT the disk image as well.

Until I upgraded to Lion. The above process is still working per se, however it is PAINFULLY slow! A 50MB backup can take up to AN HOUR! Searching around on the net, advised that there could be an issue with Spotlight as well, however this is not the case for me, since Spotlight finished a new index in under 30 mins.

While testing, I tried to copy a 200MB file ONTO the Time Machine disk (mounted from the sparseimage automatically by Time Machine) and it took OVER 3 hours! I then stopped Time Machine (which tool half an hour) and mounted the image via Finder and the same file copied in 4 secs!

I really don't get it; I bet both times the mounted Volume runes under the same protocol, AFP via TCP, why is it behaving so strangely when auto-mounted by Time Machine?

Next, I read about tmutil has been added to Lion upwards and that setting a mounted disk image as destination is now supported by TM. So I went on and mounted the image via the Finder, set the mounted Volume as the time machine destination via tmutil and gave the backup a try. The process went away really fast, was extremely pleased!

However: As soon as the backup finishes, the Volume EJECTS and TM reports that the next backup is to be made "when the backup disk is connected"! It will NOT mount the image by its own, neither to backup or to access old backups. But wait, there's more: Be selecting "Backup now" via the TM menu item, the Volume MOUNTS INSTANTLY and the backup takes place, with no issues whatsoever!

What I need is a TM that backs up quickly over the network and mounts its disk automatically.

Well, I guess if you had the time and will to read the above text, you may try to help, if you have a clue, that is...

Thank you.