I don't know enough about OpenElec to comment usefully on this, but someone else here might have some clue. I don't think overclocking is the answer.
The Raspberry Pi specific forum on OpenElec.tv is probably the best place to look for answers: http://openelec.tv/forum/124-raspberry-pi
If you find a solution, please drop a link here! Someone at some point in the future will appreciate you for it.
Edit: This problem seems of a similar ilk: http://forum.kodi.tv/showthread.php?tid=246044