Excel 2007 conditional formatting slows things down, but only on 1 workbook

Hi all,

I am having a strange issue with conditional formatting slowing down the opening of a workbook. However it is only one specific workbook. I have Googled extensively the issue and can see several suggestion sand explanations, but none seem to fit my problem exactly.

My spreadsheet is a list of codes that changes regularly (monthly) to maintain backwards compatibility codes are never physically deleted, but they are archived. So I have a cell that determines the status of a given code, and I bulk load these codes into a new sheet every month and use conditional formatting based on the status column to colour archived values.
My problem is that this months version, takes around 30 seconds to 1 minute to open the file. Yet least months (and previous versions) open almost instantly.
This is the conditional formating on this months sheet that is slow
Formula = $G2 = "D" Applies to =$A$2:$G$40334

This is the formula from last moths sheet that opens instantly
Formula = $G2 = "D" applies to =$A$2:$G$42990

As you can see, the workbook that opens instantly actually has a few thousand more cells formatted than the slow one.

I have proved (to myself at least) that it is the formatting thats causing this, by deleting the conditional formatting and reopening the workbook. When i do this, the previously slow workbook now opens instantly.

This is driving me crazy, and suggestions would be most welcome