Custom Editor: Weird reset when clicking Play

I'm creating a custom editor and I'm having a weird issue.

When I change the values in the custom editor and click Play, the values get reset to their previous values... But... If I make a change in the custom inspector Script (any change, like deleting a character, then adding it again), then re-save, and then change the values in the inspector, then these values act as expected and don't get reset when I then click Play.

I'm using Unity 4.1.5f1 on a OSX 10.8.4.

Here is a simplified version of the custom inspector (I'm also using Lists of a wrapper class containing a basic int[] arrays), but the reset occurs also with these basic IntFields:

 // MachineConfig.cs
     
     using UnityEngine;
     using System.Collections;
     using System.Collections.Generic;
     
     [System.Serializable]
     public class MachineConfig : MonoBehaviour {
         public int     symbolHeight;
         public int     symbolWidth;
     }


 // MachineConfigInspector.cs
     
     using UnityEngine;
     using UnityEditor;
     using System.Collections;
     using System.Collections.Generic;
     
     [CustomEditor( typeof(MachineConfig) )]
     
     public class MachineConfigInspector : Editor {
         
         public override void OnInspectorGUI () {
                 EditorGUILayout.BeginHorizontal();
             EditorGUILayout.LabelField("Symbol Width:", GUILayout.Width(80));
             machineConf.symbolWidth = EditorGUILayout.IntField(machineConf.symbolWidth, GUILayout.Width(40));
             machineConf.symbolHeight = EditorGUILayout.IntField(machineConf.symbolHeight, GUILayout.Width(40));
             EditorGUILayout.EndHorizontal();
           }
  }

...I also tried adding:

 if (GUI.changed) {
     EditorUtility.SetDirty(machineConf);    
 }

But it doesn't seem to help...

Any help about why this is happening will be much appreciated :)

Thanks!