#66, #67, #68 and so on can be cracked within seconds using Kangaroo. I'm pretty sure there are bots watching those addresses.
Is placing transaction (with transaction fee enough to be included in next block) in the last second before new block will be mined will help?
I'm not sure, someone else with more knowledge than I will be able to better answer you.
Although that's something I also thought of doing...
