could that cause the drop in kh/s?
possibility
Now you say you got guiminer working? cgminer would be better and its really easy to use.
http://ck.kolivas.org/apps/cgminercgminer-3.4.2-windows.zip
open up cgminer.exe then it will ask you for your pools stratum or gatework url. pefered use the stratum url with port number ex: myguild.com:3333
then it will ask you for your username and then pass. ex: myworker.1 password: 1
then you will be all set to go. once you get that up you can see the temps you are getting and other info.
i could possibility set it up for you. You could run Teamviewer and PM me your id number and password it gives you.
http://www.teamviewer.com/en/index.aspx