It sounds to me like your seat post might be not be the right size for your frame. That gap in the back seat lug should not close up all of the way, it should be open evenly from top to bottom.
__________________
|^^^^^^^^^^^^^^| ||
|......GO.BROWNS........| ||'|";, ___.
|_..._..._______===|=||_|__|..., ] -
"(@)'(@)"""''"**|(@)(@)*****''(@)