A very low magnesium level could cause a lot of extra calcium and alkalinity consumption, but that's extremely rare. There's nothing in your results that makes me think a new magnesium kit is worth getting, but if you're very worried, it wouldn't hurt to get a result. I suspect that you are seeing testing problems or limitations, which is a very common issue.
A drop of 3 dKH (from 9.5 dKH to 6.5 dKH) corresponds to about a 22 ppm drop in calcium, which is close to our testing limits. There are some processes that will consume alkalinity but not calcium, though:
http://reefkeeping.com/issues/2004-12/rhf/index.htm
The Hanna alkalinity meter might be a good investment if you are having trouble detecting color changes. Personally, I think that the consumption rate is low, which makes our test kits a bit marginal in dealing with changes in calcium level. I'd get a 2-part, or make one, and dose according to the alkalinity test results, and see how that goes.