The standard DID chain on my 550 has just 3K miles on it, having been replaced a year and a half ago along with both sprockets. The first 2000 or so miles it didn't require much retensioning, but lately I've been having to adjust the tensioners every 400 miles or so. Not unexpected. This last time I adjusted it I noticed that different parts of the chain seem to have slightly but noticeably different tension, something I never made note of in the past. By this I mean that if you check the tension in the correct spot, then rotate the tire a bit, the tension changes somewhat. Is the drive sprocket slightly out of round? What else could be the culprit? Chain and sprockets seem in good shape, with minimal evidence of wear.