Error caching, Could not cache resource

Hi, I'm getting some errors in the modx logs, the first two just happened for the first time, the last one "could not cache resource" happens about 4-5 times per day. Clearing the cache doesn't seem to have any effect. Also deleted everything under cache and still have these errors occassionally.

[2011-09-20 22:18:37] (ERROR @ /connectors/element/index.php) Error caching lexicon topic lexicon/en/core/default
[2011-09-20 22:20:20] (ERROR @ /connectors/element/index.php) Error caching action map mgr/actions
---cleared cache in manager---
[2011-09-20 22:55:54] (ERROR @ /index.php) Could not cache resource 23

modx revo 2.1.3-pl
php 5.3.3, running as dso
no opcode cache (turned off APC due to other errors)

thanks for any help or ideas,
---avibodha

What is Resource 23?

I'm assuming it's document id 23.
I get 5-6 of these a day with different document id's. But all the rest of the pages (hundreds) are in the cache just fine. I just checked and there is a 23.cache.php created a few hours ago so it did cache that page eventually. Just haven't seen these errors on an installation before. Any ideas where to look?

(Thanks for making ModX so great BTW!)