State Management :: Cache Always Removes Itself?

I have some weird problem. We're using Windows Server 2008 R2 x64 (8 Cores and
8 GB RAM) with IIS 7.5 and ASP.NET MVC 2.

I always cache (simple) stuff via the context cache and it seems like 9 out of 10 immediate page refreshs the Cache["MyKey"] is always null, even though there's no memory limit set on the pool and the server has lots of free memory.

I add expiring data via:

[Code]....

When just doing: Cache.Insert("MyKey", myObject); or Cache["MyKey"] = myObject; I get the same result (cache is almost always null for that key).

As you can see I added a callback, which writes the CacheItemRemovedReason to a text file, and the text file says CacheItemRemovedReason.Removed for MyKey. The doc for CacheItemRemovedReason.Removed says, that I call Remove/Insert on it, even though in my whole project there's no "Remove"-calls, just simple if(Cache["MyKey"] == null) {Cache["MyKey"] = ...} stuff.

I tried adding:

<caching><cache disableMemoryCollection = "true" disableExpiration = "true" privateBytesLimit="0" percentagePhysicalMemoryUsedLimit="90" privateBytesPollTime="00:02:00" /></caching>

to my web.config file in the System.Web-section but nothing changed :(

why Cache["MyKey"] is almost always null?