Timemachine weirdness and corruption

I've been noticing issues with disk utility reporting errors in the TM backup when I run Verify Disk. Repair Disk never seems to work. The disk itself seems fine as I've used it with another partition without problem. In the past I've erased the disk, repartitioned and setup TM to backup again. After a few days, the problem re-occurs.

Last check was to switch from a FW800 connection to a USB2.0 connection to see if that might be the issue. However, checking today the same result: errors reported when Verify Disk. The backup partition wouldn't remount until I'd turned off the disk and back on again a couple of times. Having done that and seeing that the backup partition mounted, I re-ran Verify Disk and it reported all as good!

I find this quite confusing: there seem to be a number of (unanswered/unexplained) threads relating to TM corrupting its backups. Could this be a genuine disk error? I'm reluctant to buy another one to find the same problem. It "smells" like a software problem, not a hardware one, i.e. a problem with TM.

Could it be a problem with putting the iMac to sleep? I religiously check that it isn't doing a backup when I do so - although I know it shouldn't cause a problem - but the machine does wake itself up occasionally to check for emails etc (no idea why, I don't want it to do that!)

It's not configured to put drives to sleep either.

thanks

Andrew