I have just tested both boards that was returned, there is absolutely nothing wrong with them - as I suspected. I used a Transcend class10 8GB card, and ran several logs on them. Examples below.
That leaves us with the logical conclusion that it must be something in your environment or method. It could be a lot of things; high voltage power lines nearby, eletric train rails, static from your logging container, noise from appliances in your home, noise from Cellphones or other personal electronics, etc. Do you happen to live right near a cellphone tower or a power station/hub?
Here are the mag plots from both boards:
Board1 (the first one you bought)
Board2 (the one I sent as replacement)
I will make that refund later today, but I will deduct the standard 40 euro examination fee, as per my terms and conditions.