It's simple: your user typed wrong.
Convert.ToInt32 tries to convert eth string, but if it finds "Hello", "123?", or "6.7" if can't convert it to an integer values, so it throws an exception.
Instead of using Convert, use Integer.TryParse:
Dim PStab As Integer
If Not Integer.TryParse(P1Stab1.Text, PStab) Then
...
Return
End If