>Just a supposition:
>If the new instance fails to initialize properly it may mean that the currently running instance is in some way blocking access to resources that the new instance needs at startup? And if the problem only happens when the site it busy (i.e. with active hits and sessions) then maybe it's just the session that is blocking the relevant resource......

It also happens when the site is having almost no user. But, it may well be that a hit is in progress.

Could you elaborate more on the session issue? I am not sure what you mean.

>Are you logging (or inadvertently suppressing) any errors that might occur at startup ?

Where would that be?

>Also, IIRC the message on the client was something to do with 'waiting' i.e. - *not* 'page not found'. That implies that the site is not completely inactive and that the client is hitting the new instance in a partially spooled-up state ( or, possibly, still hitting the old instance in a partially shutdown state).

It is probably what it is. But, if I wouldn't do anything, it would take a very long time before the site starts responding again or not at all.