Please do start a new thread in the Encore forums. Even though you will find several threads regarding pgc errors, they became complex, and a new thread will be more helpful to you.
While one CS4 user reported a pgc error, I do not think it is the pgc error that perplexed a number or us in CS3 only. (Did not happen to anyone that we can identify in CS2. Best guess was a combination of hardware and software for particular users. The exact same assets and project worked fine for users that never experienced this problem.)
Do not attempt to isolate the offending file or to do general troubleshooting until you have posted in Encore and tried some workarounds. It has generally proven to be a bottomless pit.
There are several workarounds, and the error has different workarounds depending on whether it is a motion menu, a video asset, or a slideshow.
When you do follow Stanley's suggestions, please list all aspects of your:
3.) Encore Project
These details will likely help others help you.