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First I wish to thank JHSOWTER for pointing out that my initial session timeout detection logic was flawed. That really sent me back on the right track.
So the problem was that my session was timing out due to application pool recycling.
The standard SessionTimeout solution would not work because I am on a shared hoster who controls the application pool timeout.
The SOLUTION was to add the following lines to the Web.Config file (within the <system.web> tag):
<system.web>
<sessionState timeout="60" cookieless="false" mode="StateServer" /> <machineKey ... />
To generate my machine key tag I used this tool: http://aspnetresources.com/tools/machineKey
After those changes all my problems went away.
Again thanks a lot for the help.
I have googled around for this and found a fair few people are having similar behaviour because of the AVG Link Scanner.
Firefox __doPostBack not working after idle time
http://forums.asp.net/post/4021595.aspx