Proxy failover doesn't appear to work properly using ie9

I'm testing proxy failover using an automatic configuration file.  Proxy.pac file is below.  I've shut down the first proxy to test the failover.

With ie8 the failover to next proxy works as followed.

proxy2 is used within about 30 seconds and all url lookups after this time are through proxy2 and are are pretty quick. This is how proxy failover should work.

When I test with ie9 it works as follow

failover to proxy2 take 30 -45 seconds and all following url lookups take 30 -45 seconds.

Has anyone else tested proxy failover using ie9 and do you get this behaviour?

Thanks

I've tested this with Crome as well and the failover works as well

Proxy.pac file

function FindProxyForURL(url, host)
{

url = url.toLowerCase();
               host = host.toLowerCase();
               var resolved_ip = dnsResolve(host);

if (isPlainHostName(host))
               return "DIRECT";

else if (isInNet(host, "10.0.0.0", "255.0.0.0") ||
                              isInNet(host, "172.16.0.0",  "255.240.0.0") ||
                              isInNet(host, "192.168.0.0", "255.255.0.0") ||
                              isInNet(host, "127.0.0.0", "255.255.255.0")
                              )
               return "DIRECT";

else
return "PROXY proxy1:8080; PROXY proxy2:8080; DIRECT"