The PS3 Plex app sometimes gets into a weird state where it misinterprets the remote control (I'm using the official Sony PS3 remote control). The app gets stuck in a state where it thinks one of the buttons is being held down.
For example, it will get stuck thinking that "down" is being pressed, and the Plex will continue to try scrolling the list of media items to the bottom of the list. No amount of pressing different directional buttons will convince it otherwise. The only way I have seen to resolve the issue is to exit Plex and restart the app. It is not always "down" that gets stuck when this glitch occurs, but it is the most common from my own experiences.
Please note, it is not the remote control that is going haywire, but Plex interpretation of what remote control is doing (i.e. by hitting the "home" button, I can scroll through the PS3 XMB just fine, it doesn't try to go to the bottom of the XMB menu). This problem has been in the app for as long as I have been using it (since about January), and is still present in the most recent 2.6.5 (June 2 update).