This appears to be a bug in Lion’s firewall. Is it enabled on your system?
In this MacRumors thread (DNS problems after updating to Mountain Lion (10.8)), a possible workaround is discussed:
Try reducing MTU size.
System Preferences > Network > WiFi > Advanced > Hardware > Manually > MTU: Custom > 1300
Worked for me.
Could you check whether reducing the MTU size mitigates your problem?