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I havent had this with Sedo (I don't use them) but I have had this on name drive, my traffic dropped to less than a 100 a week total now its moved back up to nearly 1000 a month. I think links die off in google / yahoo index builds and sites deindexed and it kills them. The stats offered by parking are useless for diagnosis, as you can't see the traffic origin to work out whats lost.
Maybe sedo will add an 'incoming traffic' section, I'd be tempted by that, ofcourse it would be a conflict but still its little things that sway a consumer
One of my niche sets (4 in the same niche) has been its CTR (mini sites) drop from 15% AVG to less than 1% with the recent google changes. I'm more inclined to say the same as you