Part file opens with additions suppressed

I've been working on a SolidWorks part file that continues to have my last additions (best that I can tell, that is the common factor for the suppressed features) open in a suppressed state. I can unsuppress the features and continue to edit the part file, but then the next day I seem to have to unsuppress what was added.

File history:
The file was received from my customer, so not much background info there. The file is of a complex mold and consists of one large imported surface that has many surfaces knit (for lack of a better description) together and several imported solid bodies that are essentially miscellaneous closed surfaces. I've been successful in being able to edit/add to the part file where I need to do the work, so that doesn't seem to be an issue.

Info and Experimentation:

The part file has one default configuration.
I can open the file, unsuppress the features, rebuild and save the file, close, re-open the file, and the "latest additions" are no longer suppressed.

I can do simple additions and modifications, save, close and re-open and there doesn't seem to be unforeseen suppressions happening.

But if I add or edit the file all day, rebuild and save, close and re-open the file the next day, there is suppression again of what seems to be what I worked on the day before.

I did a simple add/edit, saved, closed, reboot, re-opened and couldn't see the problem with minor changes.

Additional Info:

SolidWorks Professional 2010 x64 Edition, SP4.0

Windows 7 Professional 64 bit

4 GB RAM

SW part file size: about 50MB

So... anyone know what gives here?

I've only been running into this with this part file as far as I'm aware.

SolidworksParts And Features