index function error using match function to get data from a cell in an array

Hi All,

I am using the below function to obtain data from a single cell in an array.

=INDEX(AD13:AP27, MATCH(F4,AD13:AD27), MATCH(F12,AD13:AP13))

The function works 80% of the time. It occasionally goes brain dead and selects the wrong column. The function resides in Cell I13 on tab G01.

Can someone help me? I believe the problem may exist within Excel.

Thanks,
Mark