Have an external website which opens fine on some PC's, yet seems to time out (or symptoms of timing out, but never actually does) on others.

Seems to only affect (some) of our newer HP Pro 3305 MT Workstations. All of which are running Win7 32bit SP1 with all updates. Older PC's (Win7 32bit SP1 & WinXP) are unaffected.

Using Google Chrome & Firefox makes no difference. Opening the website in IE9 Compatibility Mode has exactly the same symptoms.

All PC's are on the same local network (Workgroup) using the same DNS server & gateway (inhouse) on the same internet connection, on the same subnet. There is no proxy server, no content filtering, no load balancing etc etc. Only group policy in effect (locally) is for Update scheduling. Local firewalls are all the same (Kaspersky WP4) and our external facing firewall has no IP specific settings.

I have no control over the external website, traceroute shows the same destination on all PC's. It is a fairly popular website in our industry (Horticulture) and i'm not aware of any other people (even other sites within our sister companies) with the same problem.

Update: Used Fiddler2 to monitor the HTTP request, seems its not getting fulfilled for some reason?!

Request sent:

GET http://www.rhs.org.uk/ HTTP/1.1
Host: www.rhs.org.uk
Connection: keep-alive
User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.47 Safari/536.11
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-GB,en-US;q=0.8,en;q=0.6
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

Log from Fiddler 2 of the request:

This session is not yet complete. Press F5 to refresh when session is complete for updated statistics.

Request Count:   1
Bytes Sent:      567        (headers:567; body:0)
Bytes Received:  0      (headers:0; body:0)

ACTUAL PERFORMANCE
--------------
ClientConnected:    17:02:33.720
ClientBeginRequest: 17:02:39.118
GotRequestHeaders:  17:02:39.118
ClientDoneRequest:  17:02:39.118
Determine Gateway:  0ms
DNS Lookup:         0ms
TCP/IP Connect: 46ms
HTTPS Handshake:    0ms
ServerConnected:    17:02:39.165
FiddlerBeginRequest:    17:02:39.165
ServerGotRequest:   17:02:39.165
ServerBeginResponse:    00:00:00.000
GotResponseHeaders: 00:00:00.000
ServerDoneResponse: 00:00:00.000
ClientBeginResponse:    00:00:00.000
ClientDoneResponse: 00:00:00.000


RESPONSE BYTES (by Content-Type)
--------------
~headers~:  0

Log of a successful request from a working PC (done this morning, excuse the timestamps being different from above):

Request Count:   1
Bytes Sent:      493        (headers:493; body:0)
Bytes Received:  20,413     (headers:525; body:19,888)

ACTUAL PERFORMANCE
--------------
ClientConnected:    08:22:47.766
ClientBeginRequest: 08:22:47.766
GotRequestHeaders:  08:22:47.766
ClientDoneRequest:  08:22:47.766
Determine Gateway:  0ms
DNS Lookup:         26ms
TCP/IP Connect: 30ms
HTTPS Handshake:    0ms
ServerConnected:    08:22:47.828
FiddlerBeginRequest:    08:22:47.828
ServerGotRequest:   08:22:47.828
ServerBeginResponse:    08:22:48.905
GotResponseHeaders: 08:22:48.905
ServerDoneResponse: 08:22:48.905
ClientBeginResponse:    08:22:48.905
ClientDoneResponse: 08:22:48.905

    Overall Elapsed:    00:00:01.1388020

RESPONSE BYTES (by Content-Type)
--------------
text/html:  19,888
~headers~:  525

So my question has evolved into:

What is the difference between the 2 requests and how do I determine why 1 PC is not getting a reply to it's GET request?