You're probably not using GPU acceleration and 2.6Ghz doesn't cut it for 1080p. I had to overclock my media center's 4850e to 3.0Ghz in order to play 1080p perfectly because FFDSHOW doesn't support GPU acceleration. I guess 2.8Ghz is enough, but i set it to 3.0 just in case.

I was getting audio/video desync with 1080p, everything 720p was just fine.