We have spent a lot of time over the last month trying to find a full set of reproducible instances of this issue.

As far as we have been able to ascertain it is something that happens more so on Internet Explorer than other browsers.

IE - happens on Many Systems
FIREFOX - happens on SOME computers
CHROME - have not seen it happen - but anecdotal reports from one client showing it.
OPERA - argh seriously why do they bother
SAFARI - have not seen it happen - but anecdotal reports from one client showing it. 

Incidentally - it also seems to be somehow connected to the FIREWALL/ROUTER/ISP connection.

Have reports of it happening on one computer ONLY when its connected using Mobile Broadband.

To be honest it kinda of feels reminiscent of some of the old issues that used to plague asp.net's post back system in 2.0 with incorrectly configured firewalls stripping some of the postback data.

Would love to find a solution to this issue - we built a recently that was designed to feature the postback system - which the client loves - BUT we are almost at the point if needing to shut the whole lot down - since it seems to fail on so many systems.

Westa