hi, Elie !

there is a wise-tricky procedure [posted by KL in spanish ng sometime ago]...
[and with minor changes based on Thomas Jansen -1999- post]...
that let's you to preserve the undo-levels, and you might want to give it a try...

if any doudbts [or further information]... would you please comment ?
hth,
hector.

assuming 'a cell' es [B1] and you need it to 'flash' if it's value is greater than 5

1) select 'B1'... - [menu] format / conditional format...
[formula]: - =(b15)*(mod(second(now()),2)=0)
[format]: - apply formats as needed/wished/...

2) copy/paste the following code-lines:
===
a) in a general code module:
===
Option Private Module
Public Sequence As Date
Sub StartBlinking()
Sequence = Now + TimeSerial(0, 0, 1)
Worksheets(1).Range("b1").Calculate ' modify/adapt/... worksheet's index/name as appropriate
Application.OnTime Sequence, "StartBlinking"
End Sub
Sub StopBlinking()
On Error Resume Next
Application.OnTime Sequence, "StartBlinking", Schedule:=False
End Sub
===
b) in 'ThisWorkbook' code module:
===
Private Sub Workbook_Open()
StartBlinking
End Sub
Private Sub Workbook_BeforeClose(Cancel As Boolean)
StopBlinking
End Sub