Just a couple of thoughts with respect to your predicament.

I am guessing that you are using one of the peeps that lays flat in the string such as a shurz-a-peep or a no loose peep. When you draw the peep does not come completely vertical, leaving you with an oval as opposed to a round sight picture.
If this is the case, I would say ditch it, and go for something that is set at a downward angle at rest. They become perfectly vertical at full draw giving you a nice round sight picture.

As for the varying sight picture my guess is that you are leaning your head in and out, resulting in the apparent changing size of the sight housing.