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Do you have a memory card in there? This might seem a bit odd but I have a 30D, had exactly the same issues on longer exposures, which stopped when I removed the card. I use BackYard Eos and download direct to pc without the card in the camera. I believe electric type artifacts get introduced somewhere in the storage process.
I would habitually do 10 to 20 min subs so it was a big issue, now resolved
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