Drupal 6 drops text below teaser on multiple re-edits.

Drupal Version: 6.1
PHP Version 5.2
Theme: Ability (Slightly Modified Salamander) with some CSS changes only
Editor module: Nicedit
Client OS: WinXP
Client Browsers: Firefox and IE
Server OS: Linux
Server: Apache, Version not known.

Steps to reproduce:
1) Create a text file of any kind whose content type is full html.
2) Editor can be enabled or disabled (which is why I'm posting this here and not under the module)
3) Uncheck "include summary in full view".
4) Add either the break comment or use the button to create summary at cursor.
5) Save.
6) Re-edit.
7) Repeat steps 5 and 6 until problem appears (usually within 4 to 5 re-edits)

Behavior observed:
Everything below the teaser disappears. If saved, additional content is lost. Behavior is pernicious once it's triggered, so that content can no longer be edited. Only workaround is to create a new content object and fill that with the contents of the old one, which is no longer available at the markup level. Object does display all content for the node as long as no further editing is attempted.

Behavior expected:
All content should remain available for editing. Calling edit should not truncate content.

Parallel Problem:
If the "Include Summary in Full View" is checked, repeated editing as per 5) and 6) above will eventually trigger repetitions of the teaser within the content body. Once this behavior begins, each edit adds another instance of the teaser.
Expected behavior:
I'm unsure how this function is intended to work, but it should not repeatedly add instances of the teaser to the remainder of the content.

Note: This is my first bug report on Drupal itself, and I was unsure what component to identify. I'm not even certain the problem is related to Drupal core. It's a critical issue, however, and the sort of thing that doesn't show up until real world use demands repeated iterations of a an apparently working function. If it belongs elsewhere, can someone put it in the appropriate place. If I find the time in the next few days, I will attempt to reproduce this in Garland with the WYSIWG editor module turned-off. However, I'm massively busy right now and have committed the site to production, a possibly ill-advised decision. ;-(!