Simple VBA script to flag transactions over $4000 that follow a pattern - excel

Question

I'm trying to write a simple script that compares and flags similar transactions (rows) and pastes them at the bottom of the sheet. The transactions which are to be flagged should meet the following criteria.
The $amount in the transactions is greater than 4000 or less than -4000 (column 11)
The two transactions being compared have the same part number (column 3)
Two transactions with similar dollar amounts (between 90-110% of each other) and opposite in number sign
Sub checktrans()
Dim newLastRow, rowcount As Long
Dim row, row2, amountcol, partnumcolcol As Integer
amountcol = 16
partnumcol = 3
rowcount = 27307
newLastRow = 37309
For row = 1 To rowcount
For row2 = 1 To rowcount
If Cells(row, amountcol) > 4000 Or Cells(row, amountcol) < -4000 Then
If row <> row2 Then
If Cells(row, partnumcol) = Cells(row2, partnumcol) Then
If Abs(Cells(row, amountcol)) > 0.9 * Abs(Cells(row2, amountcol)) And Abs(Cells(row, amountcol)) < 1.1 * Abs(Cells(row2, amountcol)) Then
If (Cells(row, amountcol) < 0 And Cells(row2, amountcol) > 0) Or (Cells(row, amountcol) > 0 And Cells(row2, amountcol) < 0) Then
ActiveSheet.Rows(row).Copy
ActiveSheet.Rows(newLastRow).PasteSpecial xlPasteAll
newLastRow = newLastRow + 1
ActiveSheet.Rows(row2).Copy
ActiveSheet.Rows(newLastRow).PasteSpecial xlPasteAll
newLastRow = newLastRow + 1
End If
End If
End If
End If
End If
Next row2
Next row
End Sub
I wrote the code above, and it seems to work for a low number of rows (below 500), but when the number of rows exceeds 27000 it goes into a never ending loop that keeps pasting new rows onto the sheet. It also posts each couple of transactions twice, which I understand is flaw in the logic which I have to work out as well.
P.S I am a giant noob when it comes to this, haven't programmed much before, and I'm just learning now to make my life easier.

Answer 1

First thing you can do is to start second loop from the point where first loop is currently. Like For row2 = row + 1 to rowcount. You checked previous records already. This will also fix problem with duplicates and you can delete If row <> row2.
Second, is to use Application.ScreenUpdating = False at the beginning of the macro and Application.ScreenUpdating = True at the end. This turns off screen updating while your macro is running and can be a huge improvement in performance.
At the end you can join all Ifs into one using And, however I don't know if this will improve performance.