diff --git a/OpenSim/Framework/Cache.cs b/OpenSim/Framework/Cache.cs
index 79e20fc88e..31cab4a021 100644
--- a/OpenSim/Framework/Cache.cs
+++ b/OpenSim/Framework/Cache.cs
@@ -199,7 +199,14 @@ namespace OpenSim.Framework
     //
     public class Cache
     {
+        /// <summary>
+        /// Must only be accessed under lock.
+        /// </summary>
         private List<CacheItemBase> m_Index = new List<CacheItemBase>();
+
+        /// <summary>
+        /// Must only be accessed under m_Index lock.
+        /// </summary>
         private Dictionary<string, CacheItemBase> m_Lookup =
             new Dictionary<string, CacheItemBase>();
 
@@ -320,19 +327,19 @@ namespace OpenSim.Framework
             {
                 if (m_Lookup.ContainsKey(index))
                     item = m_Lookup[index];
-            }
 
-            if (item == null)
-            {
+                if (item == null)
+                {
+                    Expire(true);
+                    return null;
+                }
+    
+                item.hits++;
+                item.lastUsed = DateTime.Now;
+    
                 Expire(true);
-                return null;
             }
 
-            item.hits++;
-            item.lastUsed = DateTime.Now;
-
-            Expire(true);
-
             return item;
         }
 
@@ -385,7 +392,10 @@ namespace OpenSim.Framework
         //
         public Object Find(Predicate<CacheItemBase> d)
         {
-            CacheItemBase item = m_Index.Find(d);
+            CacheItemBase item;
+
+            lock (m_Index)
+                item = m_Index.Find(d);
 
             if (item == null)
                 return null;
@@ -419,12 +429,12 @@ namespace OpenSim.Framework
         public virtual void Store(string index, Object data, Type container,
                 Object[] parameters)
         {
-            Expire(false);
-
             CacheItemBase item;
 
             lock (m_Index)
             {
+                Expire(false);
+
                 if (m_Index.Contains(new CacheItemBase(index)))
                 {
                     if ((m_Flags & CacheFlags.AllowUpdate) != 0)
@@ -450,9 +460,17 @@ namespace OpenSim.Framework
                 m_Index.Add(item);
                 m_Lookup[index] = item;
             }
+
             item.Store(data);
         }
 
+        /// <summary>
+        /// Expire items as appropriate.
+        /// </summary>
+        /// <remarks>
+        /// Callers must lock m_Index.
+        /// </remarks>
+        /// <param name='getting'></param>
         protected virtual void Expire(bool getting)
         {
             if (getting && (m_Strategy == CacheStrategy.Aggressive))
@@ -475,12 +493,10 @@ namespace OpenSim.Framework
 
             switch (m_Strategy)
             {
-            case CacheStrategy.Aggressive:
-                if (Count < Size)
-                    return;
+                case CacheStrategy.Aggressive:
+                    if (Count < Size)
+                        return;
 
-                lock (m_Index)
-                {
                     m_Index.Sort(new SortLRU());
                     m_Index.Reverse();
 
@@ -490,7 +506,7 @@ namespace OpenSim.Framework
 
                     ExpireDelegate doExpire = OnExpire;
 
-                if (doExpire != null)
+                    if (doExpire != null)
                     {
                         List<CacheItemBase> candidates =
                                 m_Index.GetRange(target, Count - target);
@@ -513,27 +529,34 @@ namespace OpenSim.Framework
                         foreach (CacheItemBase item in m_Index)
                             m_Lookup[item.uuid] = item;
                     }
-                }
-                break;
-            default:
-                break;
+
+                    break;
+
+                    default:
+                        break;
             }
         }
 
         public void Invalidate(string uuid)
         {
-            if (!m_Lookup.ContainsKey(uuid))
-                return;
+            lock (m_Index)
+            {
+                if (!m_Lookup.ContainsKey(uuid))
+                    return;
 
-            CacheItemBase item = m_Lookup[uuid];
-            m_Lookup.Remove(uuid);
-            m_Index.Remove(item);
+                CacheItemBase item = m_Lookup[uuid];
+                m_Lookup.Remove(uuid);
+                m_Index.Remove(item);
+            }
         }
 
         public void Clear()
         {
-            m_Index.Clear();
-            m_Lookup.Clear();
+            lock (m_Index)
+            {
+                m_Index.Clear();
+                m_Lookup.Clear();
+            }
         }
     }
-}
+}
\ No newline at end of file