I also don't know why that problem occours, I have one solution in the first page that solves 90% of the cases.
that is create full clone and delete all the old ones. I had one PC were xp doesn't work at all, so I had to use win7, however I believe this should be related to the router or ISP, because that happen in a different router and ISP than the rest of my pcs were I have no such problem.
Solution to this, I really don't know.. I already google for it, many people had this problem.. open support ticket maybe lol
A good router could make the difference.
If you are using a cable modem as DHCP server for your home LAN, when running 1 machine on it or 2 could be not a big problem but when running 20 VMs on it the cable modem could start to perform badly.
I see big difference when using my netgear router and when using directly the cable modem. cable modem don't have "power" to handle so many machines.