APC - User Cache Entries Vanish Upredictably

Running APC 3.0.19, PHP 5.2.4, Ubuntu Server 64 bit 8.04 APC Settings: All Default APC running fine. Op-code is caching fine. I add a few user cache entries and they appear, and live, normally. Then, I flush the user cache in code using: apc_clear_cache('user'); The user cache entries disappear, as expected. The application then goes on to repopulate certain user cache items with the same key names (and perpetual life) as those deleted in the previous step. However, these new items randomly vanish from the user cache -- lasting only a minute or two. This cycle repeats for at least 10 minutes or so... then the subsequently added keys begin to stick. This same thing happens if I use the APC.PHP control panel and flush only the user cache. IF, however, I flush both the op-code and user caches, then go on to repopulate, the user cache items do not vanish. Anyone know anything about this type of issue? Thanks.