If it's only "certain" sites - not all sites it would appear to be the site and possibly either they have some html problems or they are not compatible with I.E.6.

 

Download Firefox (another browser) and see if the same thing happens with Firefox.  I'm not advocating abandoning Internet Explorer for a different browser - just to try a different one to see what happens. 

 

BTW, since I.E.7 is the "current version" and what is really supported by Microsoft you should consider upgrading to it.