Changed system settings not saving properly - DB value is recorded, but form values and behavior don't change

This is a strange problem I've been seeing in my environment in Drupal 5 and 6. Basically, when trying to change a setting in Drupal's administration setting, the html form "sticks" to a value, rather than changing to the new value. I've seen this most commonly/recently in Drupal 5's File System and Performance settings pages. Here's an example of the what happens, step by step:

1) In admin/settings/performance, the site is currently set for "Normal" caching.
2) Click the "None" caching setting, the "Save configuration"
3) The form will post and the page will be returned notifying the user that the settings have been saved. However, the form still shows the "Normal" radio button being selected.

Now it gets strange.

4) When I look in the database, the setting is correctly recorded. However, the "Normal" behavior persists - after truncating the cache tables, generating pages yields new entries in cache_page, etc.

I've noticed this sort of behavior while trying to change the File System settings in Drupal 6, changing the tmp directory from "sites/[site-x]/files/tmp" to "/tmp" will not work.

The only major environmental difference I can think of is that we began using Aegir to manage sites, and the probably seems routine between all sites on that server. I've done some googling but couldn't find much in the way of solutions or probable causes - it almost seems as though there is some method at work beyond caching and the database layer.

Any ideas on what causes this?