How about getting rid of the macro and using some VBA in the after update event? Looking at the maceo it should work but the Argument is a little on the odd side.
To not have the combo associated with the macro you can erase the text "[Embedded Macro]" from the After Update field. Then you can create an event handler by clicking the elipses (...) and selecting "Code Builder" from the option provided. Now you will need to place some VBA in the event handler you just created. You will have to change the text in red to match your combo name and I am assuming the first column in your combo has a text field and this is the field with cust_name.
Code:
Dim rs As DAO.Recordset
If Not IsNull(Me.ComboboxName) Then
If Me.Dirty Then
Me.Dirty = False
End If
Set rs = Me.RecordsetClone
rs.FindFirst "[cust_name] = '" & Me.ComboboxName.Column(0) & "'"
If rs.NoMatch Then
MsgBox ("Not found")
Else
If rs.Bookmarkable = False Then
MsgBox "Can't bookmark this record"
Else
Me.Bookmark = rs.Bookmark
End If
End If
Set rs = Nothing
End If
edit:
Also understand that this code is saving your current record. May not work or may not be what you want in your case.
Code:
If Me.Dirty Then
Me.Dirty = False
End If