Intermittent Issues with Excel Services web parts

We have a brand new SharePoint 2007 Intranet farm running on new 64-bit hardware with lots of processor and memory. We are using Kerberos for security and have carefully followed all the written guidance from Microsoft and blogs to configuring Kerberos and Excel Services correctly. Generally everything is running great.

Currently we can add Excel Web Access web parts on SharePoint team site pages and connect them up to SQL 2005 Analysis Cubes for use with doing Pivot Tables, Graphs, etc… As part of this, we have created a test dashboard page that has six of these web parts that display upon page load. However we have noticed that we get intermittent errors when a user does things like refresh the connection or workbook, or change the filter on PivotTables. The user gets a generic error messages such as "An error has occurred. Please contact an administrator." and then their session in Excel Services is hosed up. They have to shut down IE and come back to the site in order to interact successfully with the Excel web parts again.

I have looked at the logs but just see generic messages like "EcsSoapException: An error has occurred." that don't give me something that I can really act on. Also I have checked the application event logs but didn't find anything relevant.

Any ideas on how to troubleshoot this?

Sample error from log –

08/07/2009 14:14:53.40 w3wp.exe
(0x06C4)
0x17B0 Excel Services
Excel Services Web Front End
3skc Information There was problem
processing a request on Excel
Calculation Services
http://apulu:56737/SharedServices_AI_1/ExcelCalculationServer/ExcelService.asmx
exception:
Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceSoap+EcsSoapException:
An error has occurred. at
Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceSoap.PostProcessRequest(CommandResult
result, WebMethodType webMethodType,
Exception ex, Boolean endRequest,
String sessionId) at
Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceSoap.ExecuteAsyncEndWebMethod(AsyncEndWebMethod
method, IAsyncResult asyncResult,
WebMethodType webMethodType) at
Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceSoap.EndOpenWorkbook(IAsyncResult
asyncResult, String& workbookId,
Boolean& hasOpenItemPermission)…