I'm seeing the same thing here, but I did notice a pattern:
If the page successfully loads, it's always on a new TCP connection (SYN, SYN/ACK, ACK, and go).
On the other hand, if the browser is attempting to re-use an established connection, it always seems to fail (hangs after I get a couple of ACKs from the proxy box). At least it does if the established connection has been idle for a bit.
Perhaps the conntrack timeout on the new proxy box is set too low?