Differences between Ethernet Shield versions

Hi all, i'm experiencing some troubles with official Ethernet Shield (W5100 chipset), and i'm not alone: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1238295170 http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1266450017 http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1225354009

I'd like to konw it it is related to chipset (the Wiznet W5100), or to the library. And also if an unofficial Ethernet Shield (like Xport, XportDirect, XportDirect+, or ...) may be affected by analogous problems.

Is there a comparison table of Ethernet Shields?

Regards M