[Plugin: W3 Total Cache] help diagnosing "waiting…" slowdowns

I’m running W3TC and WP latest, and other than the preload page cache problem reported earlier it seems to work well. But the biggest slowdown on my site happens in a random manner right after the browser connects, and (on Firefox) the status bar says “Waiting for [hostname]…”

This behavior occurs perhaps every dozen or so loads, can take between 5-30 seconds to clear, does not happen with particular pages, and seems to happen whether or not I’m logged in (W3TC is set to serve cached pages to non-logged in users only, though other cache settings may be involved). I’m on Bluehost Pro (which sped things up a bit), have checked CPU throttling/bandwidth and no biggies, and have checked mysql_slow_queries and no biggies. I have checked error logs, and there are only a few plugin errors, but one includes W3TC: w3_url_format() call generates a fatal PHP error…should I be worried about that one? (Full error at bottom.)

Recommendations for other ways to diagnose this maddening “Waiting…” error? I can’t believe it’s the server itself, and did not experience this before I installed W3TC, so I’m almost sure there’s a setting somewhere I don’t have optimized.

Thank you for your input!

Jim P.

http://ww.wp.xz.cn/extend/plugins/w3-total-cache/

[22-Sep-2011 14:53:09] PHP Fatal error: Call to undefined function w3_url_format() in /home2/envirolc/public_html/wp/wp-content/plugins/w3-total-cache/lib/W3/Plugin/PgCacheAdmin.php on line 213