I'm making text. When i drag the box around i lose ends of text and they start leaving ink splashes over the canvas. Best way to describe it.
I exported as PNG to see if it was you know, actually happening or if it's like an issue with processing the graphics on the canvas or something.

When i opened the PNG it was completely black, zooming in eventually revealed the white canvas and the word i was expecting to see. Can anyone explain what that's about?

Just guessing, the image you exported was a black text over a transparent canvas.
As far as I know in 0.91 they changed the previous transparent white to transparent black and on top of that rendering semi-transparent png-s is up to the renderer you use. You may get better result trying it with another image viewer or browser, importing it back into inkscape etc.

Not sure about the problem you described though, wether if it's a rendering issue of clear screening -in that case panning the object off screen and moving it back may make it gone- or
if the box refers to a flowed text's boundaries.
Would suggest using normal text then -instead of dragging the area and entering the text, just click with the text tool and type after.

Inkscape's on canvas rendering can be frustrating at times (10%-20% of the time). A work-around I use when this rendering problem kicks in is to set the display to 'outline' only:
>View
>>Display mode
>>>Outline

As for the export issue, try this:
>File
>>Export file as PNG
when this is used the 'default' setting is to export the "Page", change this to export "Selection"