Standalone ACL intermittent errors, appears to be cache-related

I'm having a hard time isolating these errors as they're very intermittent, but we often get "permission denied" errors when trying to perform various tasks in standalone CiviCRM 2.1. This is with admin accounts (the contact is in the Administrators group).

Most of the time when I delete the contents of the civicrm_acl_cache table the errors clear up for awhile.

I'll probably need to do more testing before this is fixable, but I wanted to create an issue to start tracking it.