well, congratulations... hehe that's really good on the fact that competition for that keyword could be very stiff.. I'm pretty sure you worked hard to get to that position and you deserve it.. And I agree, blog commenting is one good way to boost your rankings, as long as you don't spam anyone of course. =)
Why is it that you are always complaining about this your blog, that is how it is for all other sites. You cant expect your position in every search for a particular keyword to be static in the results page, you either go up or down.