The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
I had the same problem and it turned out to be a failed cpu heating system. Replaced it and all is well (almost). Also found that the network part of my motherboard was failing so I had to put in a new network card. BTW, my computer is a desktop that is 4 1/2 years old but still works great (except for the parts that failed