Raspberry Pi XMBC Video Lags.

I have Raspbmc installed on my RPi and it has been impossible to watch anything because of the lag. I have tried overclocking but this doesn't help at all. I have tired changing the refresh rate but Raspbmc will not let me change it to anything other than 60Hz.

This is getting really annoying. I have been working on this problem for 3 months and I can't get rid of the lag on my 1080p videos. What am I do wrong???