I had stuttering issues using SAG with my HT820's when I upgraded from a Treo 650 to a 700p, but it only happened on higher bit rate files. There's a new rev of the over-clocking app Lightspeed in public beta (
http://www.clievideo.com/downloads/lightspeed/download.html) that you could try to see if that makes the issue go away, I've successfully used it to get SAG to play well with movies and TCPMP. (The video playback still surges a bit, but that beats being flat out of the horsepower to do it at all...)
There's also several troubleshooting and quality options in SAG that you can play with to see how they affect things.