Screen Flickers On Selection Change Event?

My screen flickers when I use the following code.

Code:
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Application.EnableEvents = False
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Dim WTH As Long
If Target.Row = 19 Then
WTH = 1
End If
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
Application.EnableEvents = True
End Sub
If I comment out the code, my screen doesn't flicker.

In the "Tmp = 1" line I was using the special cells method to color constants red (without selecting anything). When I use that code instead of the "Tmp=1" line, it takes about a second to change cells, and I see the "4 Processors Calculating Message" in Excel when the code fires--even if it's not in row 19.