I am also having trouble with shared networking in build 1970. When I reboot a VM, the networking works for a few minutes, and then stops working. I usually keep two VMs up at once (one for testing in IE6, and one for IE7, both running XP). I have tried completely removing and reinstalling the Parallels Tools from both VM images. It appears that the VMs stop being able to acquire IPs via DHCP ("unable to contact your DHCP server. Request has timed out.")
I attempted killing and restarting pvsnatd, and my system.log reported:
Nov 10 10:23:29 Sunroom pvsnatd[2400]: en2: DHCP for 10.37.129.2-10.37.129.254 netmask 255.255.255.0
Nov 10 10:23:29 Sunroom pvsnatd[2400]: en3: DHCP/NAT for 10.211.55.2-10.211.55.254 netmask 255.255.255.0
After restarting pvsnatd, my VM image was still not able to get online. I restarted Windows and it began working again. I expect it will stop working in a few minutes.