Help With My Realtek RTL8139/810x Family Fast Ethernet NIC

its not so much i need help just a bit of advice really
I've been using my 20MB virginmedia broadband for sometime now but recently I've noticed after an update on my Realtek RTL8139/810x Family Fast Ethernet NIC
that my speed has gone right down before anyone says yes i know they say its up to 20MB I've done various speed tests and i always use to get 18.6MB which is really good in my opinion anyway now I've been lucky to get 12.5 or even 13
i decided today i would uninstall my Realtek RTL8139/810x Family Fast Ethernet NIC using the device manager and to reinstall it again its gone back to a version which was out in 2006
done the speed test again and its gone right back up to 18.6
now this is the part i need some advice on really should i leave my Realtek RTL8139/810x Family Fast Ethernet NIC driver on the 2006 version or should i update it back to the 2008 version

thanks for your time in advance

p.s o-yeah i forgot to say please don't say it may be because of the operating system your using because I've had the 18.6MB on windows xp home and pro also on windows vista ultimate and the one I'm using now windows 7 so i know its not because of windows thanks again