flex : Random NetConnection.Call.Failed: HTTP: Failed

We are experiencing some random NetConnection.Call.Failed: HTTP: Failed when using the flex 4 mx.messaging.channels::SecureAMFChannel

We have never had this problem in our local servers (server/client on the same machine), but when we went live, some users experienced this problem randomely (about 5% of the 800 beta users).

Lately, at last i was able to reproduce the problem every once in a while on one client machine (windows XP), in our LAN using a remote server in the same LAN (but not on other client machines who share the same LAN and are roughly the same configuration).

And when this happens:

• Using firebug / flashbug : I can see that the amf request is sent and waiting for a response.
• On the server there is nothing in the logs (no request received).
• After some timeout, the client shows the above error (http code 404).
• Then for the subsequent requests, everything returns to normal untill the error eventually happens again

(Clients are Windows XP, Firefox + adobe flash player, Server is WebSphere 7)

My questions are: is there a way to find out what's going on ? is it a packet loss ? is it because of the client browser / adobe player ?

Thank you

[Edit]

Using Wireshark (on both the client and server side), i can see that when the problem occurs no tcp connection is made: - Firebug detects an outgoing http amf request - Wireshark doesn't detect any activity on either side.

So it's defenitly Adobe Flash Plugin / Firefox plugin container, am i right ? is that a known issue ?

Setting the proxy parameters of our browser resolved the problem ("No Proxy" instade of "Automatic detection" or "system proxy").

It looks like Adobe Flash plugin gets lost when a proxy is selected.