Lines in Frame become twisted in PDF

[Doug]

I use the Rounded Rectangle tool to draw callouts around fields on
screen images, which are inserted via reference in the Frame doc. 
When I PDF a document with these callouts, the lines of the rounded
rectangles appear twisted.  For example, one half of one of the lines
of the rectangle slopes down on top, as if a portion of the line had
been scooped out.  The other half of this line slopes up, as if the
scooped-out portion has been moved onto this portion of the line.  The
entire bottom of this line is straight and normal.  Some of the
callouts suffer this problem on just one of the lines; other callouts
suffer this problem on both lines.  (The callouts I use are usually
long and narrow.)  The end of the rounded rectangles always display
fine.

If I use the square corner rectangles, this problem doesn't occur.

I'm running WinXP Pro, Frame 7.0p579, Acrobat 7.0.7 Pro, and I use the
Save As PDF function in Frame to create the PDFs.

Has anyone else seen this problem?

--Doug