How are you importing the text, and what source are you using? For example, are you using File > Get Text... or just copy and paste? Is it plain ASCII text, or is it from a formatted source, such as an MS Word file? My first suggestion would be to try a few combinations of the above to find out what works and what doesn't.
Another potential problem could be a rogue XTension. I would suggest turning off all XTensions in the Utilities > XTensions Manager and try again (you'll need to restart Quark). If it works, you'll have to isolate the problem XTension and get rid of it, or reinstall it.