Borders problems in navigation bar

[Pascal]

When making text larger, a glitch occurs with borders in the horizontal
navigation bar on these 2 (excellent) sites
http://karlcore.com/index.php (good articles)
and
http://css.maxdesign.com.au/floatutorial/tutorial0611.htm (good tutorials)

In Firefox and Netscape, some of the vertical borders disappear or grow
wider.
Opera does not loose borders but makes some wider.
IE has no problem at all! ;-!

Something with screen and pixels?
Is it possible to correct this?

Thanks to all.