I am getting this:

17/07/2007 12:31:19 Application Started ...
17/07/2007 12:31:01 Application Started ...
17/07/2007 12:30:59 Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting env ...
17/07/2007 12:30:59 Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting env ...
17/07/2007 12:30:11 Application Started ...
17/07/2007 12:29:54 Application Started ...
17/07/2007 12:29:52 Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting env ...
17/07/2007 12:29:37 Application Started ...
17/07/2007 12:25:52 Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting env ...

Although the details point to the hosting environment, I do not get it on another domain I have on the same server of that hosting service (Lunarpages).

I am not sure when did it start. I have just now upgraded to 4.5.3 to benefit from the shutdown details, but it was happening all the same when I was on 4.5.0.

Any idea what may be causing it, or how can I debug it any further?

Mitchel, Thanks for your tips.
My site is not busy at all.
The site is responding a little slower sometime, but other than that the users can’t tell the difference. Only I have a problem when I try to upload a large media file to one of my repository modules. The browser just goes blank saying it did not get response from the site, and the only log I see is again that application shutdown.
I have requested further information about my site’s ASP.NET worker process recycle settings. I also requested that they will check if the MS hot fix mentioned in http://support.microsoft.com/kb/920970 was implemented on my site. So far I did not get a clear answer. Anyway the server’s control panel says that the ASP.NET session timeout is set to 20 minutes.
The search scheduler is set to every 30 minutes, with retries every 60 minutes. I have disabled it to see if it has any effect, but the application continued to shutdown in the same manner. I also tried to disable the PurgeLogBuffer just because it was scheduled for 1 minute, but that also had no effect.
S.B.

HttpRuntime parameters were already taken care of (see my posting in http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/54/threadid/143912/scope/posts/Default.aspx)

The file I am trying to load now is relatively small, and would have been loaded even without the Httpruntime change. In any case when that timeout happens there is a very specific log for it. The case now does not produce this log.

The shutdowns happen all the time, and more frequently when I am working with the site. Any work. Not just file loading. The time between application start and shutdown is not the same all the time, it is around 1 minute.