How fast is your hashing speed? There might be a problem with your internet connection tho..
my speed is about 7 mH/s (i know, it's very slow, but I don't care)
this is my normal pc, i'm surfing, playing, ... while I mine, so I don't think it's a problem with my connection.
Known bug with newest Guiminer. Use Phoenix or older Guiminer version.
same happens in all miners I tried - guiminer (newest and older version), cgminer and phoenix.