Asp.Net GetVaryByCustomString(...) in web farm scenario

Question

We have a problem with output caching used within a web farm environment which cannot be replicated in a single node situation. An un-cached page has an un-cached parent user control which has child user controls which are cached. The time to cache is set by setting the PartialCachingAttribute on the user control's class declaration and then the VaryByCustom string is set programmatically in the user control's OnInit event. There is a gobal setting for toggling caching and a block of code which checks for a required list of query string values from the Request object before caching is attempted. GetVaryByCustomString(...) is overloaded in the Global.asax file.

The single node version correctly displays the child user control content within the parent user control but the web farm content caches incorrectly - it mixes up cached items from different calls.

It seems that the output caching mechanism is not working in the web farm scenario. Any ideas?

`<`PartialCaching(600)`>` _  
Partial Public Class classname  
    Protected Overrides Sub OnInit(ByVal e As System.EventArgs)  
    Me.CachePolicy.Cached = False  
    'only do caching if global setting is on - elided this code  
    'only do the caching if the call for this control has the correct params -   elided   this code  
                    If (Me.CachePolicy.SupportsCaching) Then
                        Me.CachePolicy.VaryByParams("none") = True
                        Me.CachePolicy.SetVaryByCustom("MyCacheKey")
                        Me.CachePolicy.Cached = True
                    End If
    MyBase.OnInit(e)
    End Sub
...
End Class


Public Overrides Function GetVaryByCustomString(ByVal context As   System.Web.HttpContext, ByVal custom As String) As String
         Try
            Select Case custom
                Case "MyCacheKey"
                'only do the caching if the call has the correct params - elided this code
                If ...
                    Return String.Format("{0}_{1}", custom, myUniqueString)
           Else
                          'return a random key otherwise we will have only one cache item for non-captured calls
                           Return String.Format("{0}_{1}", custom, Guid.NewGuid().ToString)
                        End If 
            End Select
            Return MyBase.GetVaryByCustomString(context, custom)
        Catch ex As ApplicationException
            Return Guid.NewGuid().ToString
       End Try
 End Function