Coldfusion intermittently running slowly

We recently migrated several coldfusion websites from

Windows server 2003 to 2008 and IIS 6 to IIS 7.

We are running Coldfusion 9 enterprise  64 bit in both environments.

The first couple of days ran succesfully, but then we sufferred intermittent slow responses from the websites.

The only errors reported were http 503 back from IIS and some CFLOOP and CFHTTP timeouts in our exception handler.

We have tried turning off all CF schedules and increased the queue length property on the Application Pools

The CFLOOP which is timing out looks as follows:

   <cfloop collection="#request.session#" item="local.ii">

    <cfset session[local.ii] = request.session[local.ii] />

   </cfloop>

Of course, this timeout may be a symptom of the problem rather than a cause.

Many Thanks,

Ian Bagnall.