Written In Visual Basic 2005 And Running On Windows XP SP3?

I hope someone can help with a very frustrating problem I'm trying to solve in one of our Windows Forms apps written in Visual Basic 2005 and running on Windows XP SP3.

The app is essentially quite simple in concept but there is lots of code and data, however, the problem does show itself consistently. The app does the following:

1) Startup

2) Choose Access database to load data from

3) Load the data into memory as a series of collections of objects

4) Do lots of number crunching

5) Fails when entering a particular sub for the first time with an Access Violation Exception

The most difficult aspect of this problem is it only shows itself when compiled in Release mode. It doesn't show itself when running in Debug mode either within Visual Studio or running outside Visual Studio. I assume this is due to the extra 'padding' in a debug exe which is masking the effect of bad code causing the error. The problem is made worse in that the 'standard' build target machine where the problem shows itself only has a copy of Visual Basic 2005 Express so as a result I have been debugging the app at runtime using WinDbg which is tricky but sort of fun (for a nerd!).

I have read / researched extensively on the internet for solutions and as a result have tried the following but all without success:

a) checked for any badly declared DLLs (p/invoke) but the app doesn't use a single one.

b) checked for any COM references / interop. There were two references which were rarely used so I removed them and commented out the code that referenced them but the exception was still raised.

c) I've tried adding my own trace code in the problem area and the line prior to calling the Sub runs but the first line of the Sub never executes due to the Access Violation exception being raised.

d) I have used WinDbg to set breakpoints around the problem area at runtime which has confirmed 'c' above that the sub never executes.

e) I have used WinDbg to double check the addresses of the parameters passed into the Sub are correct as they seem to correctly point to the same address as the local / module level variable that was passed in.

f) I have checked all of the extensive data loading is not eg leaving DataReader objects open etc.

g) I have tried compiling the code in VS 2008 on a different machine targeting the 2.0 framework but the bug still showed itself.