Fair enough. This might be unrelated, but are you using a .ext3flash file as proposed in http://www.nslu2-linux.org/wiki/Unslung/Ext3flash ?
Well either your stick is badly failing again, or some of the slug’s internal memory is failing. I’ve never seen such weird bugs on a rather stable piece of software.
Did you try to do some kind of memory benchmarks, or actually test the RAM of the slug? http://www.nslu2-linux.org/wiki/HowTo/TestSDRAMinRedBoot seems cumbersome, but would yield in potentially being able to rule out RAM as a culprit.