This is very interesting. However, the OP's idea of "failure" is when something goes wrong with the system -- in this case, ext3 detected an error when trying to write to its journal. It's possible that silent data corruption occurred much earlier. If the OP reads this, I'd love to see the experiment repeated, with the added step of verifying the written data every time it's written.

That is, you would dd a random bit pattern to disk. Then you would read it back and XOR it with the pristine pattern you know you wrote. If you see any difference, you've got a much scarier kind of failure than the fail-stop behavior exhibited in the post.

Also, does the OP know whether any write buffering is going on? That could explain why some writes took much longer than others -- when the write actually goes to disk, it would take much longer than when it gets cached in memory. Use the "sync" command between each write to make everything go right to disk (I think?).