Onders - 2:44 pm on Sep 13, 2011 (gmt 0)
Thanks for the responses - no, there is nothing reported in the error logs. Interestingly it seems completely random how it happens and although there are cases where we can't view the source the logs suggest that there was a 200 each time.
We've wondered about memory problems - but don't think that's it.
Likewise, have heard that PHP sometimes causes problems when a process is running. Especially happens in CMS systems.
This is really bizarre - I've got our very friendly hosting company looking at it with their developer techs.
Thanks for your inputs so far!