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 lets 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 its 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/... worksheets 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