I had a possibly similar problem, but mine didn't occur until after the edit window was open for 15-18 minutes.  Thanks to some fancy sleuthing by John Mitchell, he suggested that I set the "Page State Persistence" to "Page" and not to "Memory" as I had set it.  See below.

Page State Persistence: Select the mode to use to persist a page's state. This can either be a hidden field on the Page (Default) or in Memory (Cache).	Page Memory
Module Caching Method: Memory caching provides the most flexibility and is the highest performance method however it also consumes the most system resources. Disk caching is the least expensive on resources but does not offer any flexibility.	Memory Disk

Since making that change, and perhaps a few others (you can find John's recommendations for DotNetNuke performance settings in his fine forums at Snapsis.com), I haven't seen that problem again.

So ... check your performance settings ...