Ichat constant "communication error" when i try to video/audio chat or screen share?

I am having major problems with my macbook (late 08) when ever i go onto ichat and try to do a screen share or a video or audio chat i get the message "there was a communication error during your chat, did not receive a response from bla bla bla". My internet is fine because my sisters macbook works fine... show more I am having major problems with my macbook (late 08) when ever i go onto ichat and try to do a screen share or a video or audio chat i get the message "there was a communication error during your chat, did not receive a response from bla bla bla". My internet is fine because my sisters macbook works fine and she does about 20 video chats a day. Dont think that the problem is we are on at the same time cuz we arent. My video chats just dont work with anyone for some reason . I took it to the apple store but those idiots didnt know how to fix it they just re installed the software and sent me away saying it would work. HOW CAN I FIX THIS
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