using Powerpoints can be problematic, given the abundance of thin lines, aliased text and general overstuffed slides which do not look good on video.
The problem you refer to sound familiar – try changing the scale so it is within the safe action area.
I tend to open every powerpoint slide in Photoshop and tweak it before importing into Premiere.
You can also tweak the Powerpoint file itself before exporting BMP images – adjust font sizes and colors and layout to stay within the safe area.
keep content away from the screen edges and see if you still have this problem.
If you export an AVI, how does that look?
Mike