HTTPERR filling up, fast with Timer_ConnectionIdle

As per the KB 820729

Timer_ConnectionIdle : The connection expired and remains idle. The default ConnectionTimeout duration is two minutes.

Timer_ConnectionIdle, happens in normal HTTP protocol usage, where the client decides not to disconnect from the server because there is a good chance that it will have another request for the server either in the process of loading a page or because a client will probably click a link on a web page that will go back to the same server. By default the server will close the connection and reclaim those resources after 2 minutes of inactivity. This is nothing to worry about, it's just an informational type of entry.

I hope you do have 180 seconds of default timeout setting configured in IIS. You can use the article Configuring HTTP Server API Error Logging to completely disable HTTP.sys logging (not recommended) but there is no way to disable a particular event.

There is a very fast and easy way to completely disable it; add the following value to the registry:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters]
"EnableErrorLogging"=dword:00000000

Then stop and start the HTTP service to apply the changes:

net stop http
net start http

Source: Error logging in HTTP APIs