The problem you mentioned sounds quite abnormal and I've never heard of it before. Have you tried different power sockets for the PC and W8960? If the computer is off all the time, W8960 will be...

Martin, if it is not a driver issue, maybe they are not compatible with each other.

Maybe there is another unsecured powerline network in the same electrical meter... Anyway, secure it to test. http://uk.tp-link.com/article/?faqid=258

Maybe you can open this one. http://uk.tp-link.com/article/?faqid=258

Did you choose correct egress rule? http://www.tp-link.com/en/article/?faqid=788

Did you read this article for some hints? http://www.tp-link.com/en/article/?faqid=328