make sure your item renderers do not use event handlers in the mxml, as mxml event handlers do not use weak references and thus never get GCed.
For example: I had an itemrenderer component in mxml that looked like
<s:ItemRenderer creationComplete="OnCreationComplete">
instead I created a base class:
public class ItemRendererBase extends ItemRenderer and in the constructor added the event listener this.addEventListener(FlexEvent.CreationComplete, OnCreationComplete, false, 0, true)
for sub component event handlers such as <s:Button click="OnClickButton"/> remove the inline event wiring and instead wire the event handler in your override OnCreationComplete with false, 0, true to use a weak reference.
also make sure you manually invoke the GC collection for testing this and I recomend using adobe scout