Excel Iteration Errors

I have a large cash forecasting model with weekly data in each column.  Each column has numerous index/match formulas that look up values from other sheets in the workbook.  Each column concludes with ending cash and the next column begins with with that ending cash from the previous week.  

For example, B1 = A20, C1 = B20 and this format continues outward for 75 weeks or until the end of 2019.  So, there are literally thousands of index/match formulas and a few hundred circular references in this model.

Everything works fine until you reach ~30 columns and then the numbers start to drift.  For example, assume AC20 = 100, therefore AD1 should = 100 also.  But it doesn't, It displays 98.  However, if you press F2 and then F9 the answer will be 100.  It's the strangest phenomenon I've ever seen in Excel.  

Oddly the error pattern seems to run in a sine wave fashion.  The first column will be off -2 as in then example above and then the next column is off +4 and then -8 and then +16, etc.  This same pattern continues through the last column and the variances get very large.

I have tried numerous combinations of formula iteration tweaks in Excel Options, I have tried "precision as displayed" and I have tried rounding the formulas in the target cell and the destination cells of the index/match formulas.  Nothing seems to make a difference.

Does anyone have any idea what could be causing this error to occur in Excel?  I've never see anything like this before.