سلام اینکار بسیار ساده است
    Dim i As Integer
Dim d As Integer
Private Sub Button8_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button8.Click
i = 0
d = DataGridView1.Rows.Count
Timer1.Enabled = True
End Sub

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
If i = d Then
i = 0
End If
TextBox5.Text = DataGridView1.Rows(i).Cells(0).Value.ToString
i += 1
End Sub