Logged-in user changes while browsing a C5 site

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Has anyone else experienced this? I have an intermittent problem on a C5 (5.4.2.1) site where a registered user would login as, say, Bob and then sometime later find that they're logged in as Fred.

Have had a note from a user today to say a completely different person received a Guestbook notification than the person in its settings.

The user is being displayed at the top of the page using
<?php $u = new User();  ?> ... <?php echo htmlentities($u->uName); ?>
and the site uses APC cache. Server is running Ubuntu Lucid LTS.

surefyre