We're having a few issues with our image server.

It's a Win 2008 running IIS 7.5 and it only serves static content: images.

It has run without issues for quite a while, until recently when we disabled Output Caching, as we noticed having it enabled meant it sent no-cache host-headers to the clients (forcing them to fetch the images from the server every time)

We've read quite a bit about it, and it seems IIS just works that way - either you use Output Caching or you get to use cache host-headers.

Anyway having disabled the Output Cache, we now experience random 5 minutes intervals, where all requests just get a 503 Service Unavailable. During this period the "Files cached" performance counter staggers (neither increased nor decreased) and after the period all caches are flushed.

You might find it weird I talk about caching, since we disabled Output Caching.

The thing is we changed the ObjectTTL parameter in registry, so we cache files for 3 minutes (which has worked very well, our Disk I/O dropped significantly)

So even with Output Caching disabled, we're still caching plenty of files - if we could just get rid of the random 503 it'd be perfect :-D

We don't get any messages in the Windows event log during these 503 intervals, so we're pretty stumped as to what to do.

Any ideas are very welcome :-)