Microsoft Excel Not Responding

I created a spreadsheet approximately 10 years ago using excel 97 to keep track of employee 401(k) contributions and employer match.  The format has changed somewhat over the years but the general layout is the same.  There are 13 sheets or tabs, one for each month, and a summary sheet that accumulates YTD contributions, match, and total remittance.   In the beginning we remitted payments to the Trustee once a month so entries were made for the employees as remittances were required.  We also used the totals generated to develop entries to our accounting records.   As the plan grew, requirements for remitting payments to the trustee increased from monthly to weekly  so additional functionality was built into the spreadsheet to accommodate separation of weekly amounts on each monthly sheet while documenting the weekly accounting entries and still allowing YTD amounts to accumulate.  2 macros were added to allow this last bit of functionality.   The spreadsheet worked very well for several years until we upgraded to Office 2007.  The first time that I accessed the file after the upgrade I noticed that it took an excessive amount of time to load.  Data entry in a given cell could take anywhere from immediate up to several seconds depending on which monthly sheet was being worked on.  The months of April, July, September, and November are especially troublesome.   Attempting to run a macro on any of the trouble month sheets could make excel stop working completely.   Copying and pasting data can sometimes take 3-4 minutes to accomplish, that is when it doesn’t make Excel stop responding all together.  When these problems started I would borrow another workstation that was running an earlier version of excel and this worked for a while.  Eventually however, the same problems were encountered with the earlier version of excel.  

I am currently running Microsoft Windows 7 Professional and Office 2010 on an HP Elite 7000 PC with 4GB of Ram.  The processor is an Intel Core i7 860.

I’ve been using Excel for approximately 15 years and Lotus 123 for another 15 years prior, so I consider myself quite experienced with spreadsheets.  I am an accountant not a programmer and I’m certain that there are more efficient and competent users than I.   But I’ve never experienced this kind of performance degradation.  This has become a very frustrating and time consuming issue during certain months (trouble months above) and I get angrier with Microsoft every time I have to deal with it.  I have attempted many things to try to alleviate the problem but to no avail.  I’ve set recalculation to manual, I’ve deleted certain “If” formulas thinking that these were slowing down performance, and I’ve converted the file to an excel macro file format.   Nothing that I’ve tried seems to help.

Can someone offer some help?  I can submit a version of the spreadsheet that has been edited to remove SS#’s, names, etc.  for review.