As a best guess without actually running your code you are getting into a
recursive loop. In your change event you are changing values which will
initiate a cnage event that will ... You need to disable events while you are
making the changes... Note that I added in the .value at the end of curcell.
While value is the default it never hurts to be explicit. Also throughout
your code you have Range("??"). You are best off to explicitly declare which
sheet they are referencing.

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
 Dim w As Worksheet

Set w = Worksheets("Run1")
Set curcell = w .Range("G18")
Set curcell2 = w .Range("H18")
On Error GoTo Endit
application.enableevents = false
       If curcell.value <= 0.3 Then
           Range("H18") = 4
           Range("M25") = 6.7
           Range("M26") = 25
           Range("M27") = 75
           Range("M28") = 93.3
           Range("M29") = 0
           Range("M30") = 0
           Range("M31") = 0
           Range("M32") = 0
           Range("M33") = 0
           Range("M34") = 0
           Range("M35") = 0
           Range("M36") = 0
         
           
           
       ElseIf curcell.value  > 0.3 And curcell.value  <= 0.7 Then
           Range("H18") = 6
           Range("M25") = 4.4
           Range("M26") = 14.6
           Range("M27") = 29.6
           Range("M28") = 70.5
           Range("M29") = 85.4
           Range("M30") = 95.6
           Range("M31") = 0
           Range("M32") = 0
           Range("M33") = 0
           Range("M34") = 0
           Range("M35") = 0
           Range("M36") = 0
         
       ElseIf curcell.value  > 0.7 And curcell.value  <= 1 Then
           Range("H18") = 8
           Range("M25") = 3.2
           Range("M26") = 10.8
           Range("M27") = 19.4
           Range("M28") = 32.3
           Range("M29") = 67.7
           Range("M30") = 80.6
           Range("M31") = 89.5
           Range("M32") = 96.8
           Range("M33") = 0
           Range("M34") = 0
           Range("M35") = 0
           Range("M36") = 0
       
       ElseIf curcell.value  > 1 Then
           Range("H18") = 12
           Range("M25") = 2.1 And Range("M26") = 6.7 And Range("M27") = 11.8
           
           Range("M28") = 17.7 And Range("M29") = 25 And Range("M30") =
35.6 And Range("M31") = 64.4

           Range("M32") = 75 And Range("M33") = 82.3 And Range("M34") =
88.2 And Range("M35") = 93.3 And Range("M36") = 97.9
           
           End If
application.enableevents = true

Exit Sub
Endit:
application.enableevents = true
End Sub