So I have this wierd problem; When I watch 720p-movies they randomly start to lag. I mean at first I wasn't sure if my "setup" was able to run these types of movies, but it all worked perfectly fine for a week or so then it just started lagging again.
Im currently running a wireless connection to the PS3, might that be the problem? (Would be quite strange though seeing as it has worked perfect before)
Only one tip: When you will connect cable to PC/PS3 it doesn't mean this connection will be automatically used. You need to disable WIFI on PS3 for switching to cable connection. Then PS3 will be able to play higher bitrate movies(transcoded etc.)
I was in your same situation. Wireless just wasn't fast enough. I also ran a wired connection the PS3 for better streaming. I forced PMS to use the wired NIC for streaming so that I could have both networks up at the same time, as the wireless is my internet connetion for the laptop. It seems to work ok doing that.