Autofill memory allocation problem or?

Hi All,

I'm a beginner in vba and encountered what I beleive is to be a memory allocation problem or what?

This is the macro

ActiveWorkbook.Worksheets(2).Range("P2").Formula = "=SUMIF($M$2:$M$100000,M2,N2)"
ActiveWorkbook.Worksheets(2).Select
Range("P2").Select
Selection.AutoFill Destination:=Range("P2:P" & Range("A" & Rows.Count).End(xlUp).Row)

But after 3188 rows it stops working??? Why? Column N is filled with 1. Column M with product grades and I want to sum all grades that are equal and place the sum in column P.