Cache Broken Drupal 5.12 Site

I am working on a Drupal 5.12 site. Recently, apparently for no reason, meaning no known cause, anonymous users began seeing empty pages--no HTML content was being delivered to their browsers whatsoever. Logged in users were fine. I eventually determined that if I turn off the cache, then anonymous users are fine.

The cache that I disabled was of the Normal variety, not Aggressive. The problem remains, however, that if the cache is enabled, anonymous users get the WSOD. After I was able to get the site working for anonymous users, I began to debug what exactly the problem could be. I tried clearing the cache using the devel module and I also tried clearing it directly in MySQL:

truncate cache;
truncate cache_content;
truncate cache_filter;
truncate cache_menu;
truncate cache_page;
truncate cache_views;

After clearing the cache (via any method) the first page request I do as an anon user works, but the second fails--I get WSOD.

I then ran this test, as simple PHP code in a page I made, to see if the cache is working:

cache_set('test', 'cache', 'my testing data');
print_r(cache_get('test', 'cache'));

and the results appear to indicate that it works fine:

stdClass Object
(
    [data] => my testing data
    [created] => 1227622264
    [headers] => 
    [expire] => 0
)

So I thought to examine what's being cached exactly. I cleared the cache and then checked to make sure from the MySQL CLI:

mysql> select cid,created from cache;
Empty set (0.00 sec)

Looks clear. Then I requested a page as anon user (with caching disabled BTW) and then I saw this:

mysql> select cid,created from cache;
+-----------------------------------+------------+
| cid                               | created    |
+-----------------------------------+------------+
| variables                         | 1227626901 |
| imagecache:presets                | 1227626901 |
| emfield_implement_types           | 1227626903 |
| emimport_types_allowing_import    | 1227626903 |
| date_formats:field_expires_on:    | 1227626922 |
| date_formats:field_date:          | 1227626921 |
| date_formats:field_date_end:      | 1227626921 |
| date_formats:field_date_start:    | 1227626921 |
| google:docid-4523674103796100817: | 1227626902 |
| google:docid6452795720218076508:  | 1227626922 |
+-----------------------------------+------------+
10 rows in set (0.00 sec)

I then waited a few minutes and checked again:

mysql> select cid,created from cache;
+-----------------------------------+------------+
| cid                               | created    |
+-----------------------------------+------------+
| variables                         | 1227626901 |
| imagecache:presets                | 1227626901 |
| emfield_implement_types           | 1227627032 |
| emimport_types_allowing_import    | 1227627032 |
| date_formats:field_expires_on:    | 1227627029 |
| date_formats:field_date:          | 1227626921 |
| date_formats:field_date_end:      | 1227626921 |
| date_formats:field_date_start:    | 1227626921 |
| google:docid-4523674103796100817: | 1227626902 |
| google:docid6452795720218076508:  | 1227626922 |
| google:docid-8010916811852138370: | 1227626966 |
| google:docid8984424637152522592:  | 1227626966 |
+-----------------------------------+------------+
12 rows in set (0.00 sec)

I presume that there are actual visitors out there surfing to the site as anonymous users, and that explains why the cache is growing. I can guess as to why those particular entries are showing up because we have imagecache, CCK with a set of fields, events with dates, and we link to Google videos.

At this point, however, I am not sure in what direction to proceed with debugging this. The cache seems to be filling up, which is expected, and my test run at storing and retrieving data from the cache works. What it is that is not working I do not know.

Any ideas are appreciated.