WP-Cache taking forever to generate cache file

I am having a particularly strange problem that seems to revolve around the use of WP-Cache.

I run a number of WordPress blogs on my site, these are currently all version 2.2.1 (upgrading to 2.2.2 in a short while), each with the plugin WP-Cache enabled with a cache age of 24 hours (86400 seconds), hence database and PHP usage should be minimised (each has its own dB / subdomain). The only other plugin running is Akismet.

Recently, in the last three or four days I have noticed that pages, when I attempt to load them for the first time, take approximately 120 seconds to load. Even though the diagnostic from WordPress states this (once eventually loaded):

<!– Dynamic Page Served (once) in 0.394 seconds –>

Sometimes when I refresh the page loads correctly straight away, other times I have to wait 120 seconds again.

<!– Dynamic Page Served (once) in 0.394 seconds –>
<!– Cached page served by WP-Cache –>

Once this “cached” notice appears it seems the majority but not all of the problems disappear. I have not knowingly changed any settings on the sites to cause this problem to develop and it is strange that it is effecting all blogs in the same way.

This happens across all wordpress blogs, including those with only a few posts.

When I turn WP-Cache OFF the site loads quickly, getting the information from the DB.