Sure and this could be another reason why Hep finds the terms to be a little confusing. In these threads, we often use them as shortcuts without having to constantly add the qualifiers and disclaimers, but when you are pondering the proper use of the terms as a title, as Hep is, it helps to keep the perspective in mind.
Agnosticism is an opinion about knowledge, so it is individually applied to specific areas of knowledge, traditionally theism.
Skepticism is a process to judge truth value of any knowledge. It can only be applied after the question of agnosticism is answered. If we do not, or can not, know a subject, then there is nothing to be skeptical of. So skepticism is not on the fringes of agnosticism, it is beyond it, post facto.
So I can not be skeptical of theism unless I agree that the knowledge is knowable. But, as you say, these concepts exists in different levels of the mind. If you take agnosticism out to it's extreme, then I am agnostic. Although I believe theism as expressed in religious forms is knowable and therefore subject to skepticism, I also acknowledge that it is possible, if God is all powerful, he could exist in such a way as to be unknowable. In this case skepticism becomes as useless as religion, because something unknowable to that extreme is also negligible. That is where the rational part comes in. If it does not interact with reality in any way, then it is not worthy of efforts to understand.