Here are some things to test to see if it is a Quark problem or a font problem:
1) Is it possible that your font is damaged or a wrong type (such as bitmap)? Try a different font to see if those letters show up correctly.
2) Is it possible that you have either a missing PostScript font (you have screen but not printer portions) or Adobe Type Manager turned off, which could make things render poorly.
3) Is it possible that you are using a bitmap font instead of a TrueType or PostScript font? In that case, Quark may be making an attempt to display an enlargement of the bitmap.
4) Are you using a large, decorative font? Your font may be too complex to render clearly, especially on a curve or angle.
You may want to print a page to see if it still prints well in spite of the screen image. That would tell you if the situation is just a nuisance instead of a problem.