I believe you are on the right track. I think he is right. It is the media - I rarely if ever have an issue with a game or clip i get directly from my DSLR or a colleague who has sent me something via drop box or SD card direct.
The problem is on media I down load or am sent that is already on You Tube or another source. That being said, I can not find the media cache option in preferences in Elements. However, when I click on the Media option under Preferences in Elements I can see that the Media Cache Database has a box checked that says "clean once a month". but under preferences there is no Media Cache Option to click on - only a Media button. FYI - I hit the clean cache now button just in case.