You should always do the first backup by ethernet.. even now I wonder if it is better to connect by ethernet and do a verify of the TC backup...
See A5.. http://pondini.org/TM/Troubleshooting.html
You should also load the widget for TM.. A1 to see if anything shows up as an error in TM.
the problem being such a long backup over wireless may corrupt and takes forever to check.
I would also like you to check the log of the TC.. see if you are getting continual drop out of the AFP connection.
Please point me in the direction to the firmware version.
The latest firmware is listed on the summary page.. 7.6.1 and you cannot go back... so don't even think about that.
Summary page shows the firmware version I run.. with Snow Leopard.. always 7.5.2.. rather out of date.. but much better for Snow Leopard.
I did a quick search back but search here is pretty hopeless.
See https://discussions.apple.com/thread/4220846?answerId=19313630022#19313630022
I believe you should just try a factory reset.. Once you do check the log etc.
You might even just try a reboot of the whole network and see if the problem goes away for a while.
A reset will not erase the data on the drive.. that is only deleted if you deliberately erase.