I am hosting and playing on a G4 350MHz with 512MB RAM over 100MBit ethernet.
The thing is, if I am playing sp there can be an enormous firefight with 5-10 units involved, without this problem occuring.
In a multiplayer game with five people, all it takes to create a short freeze(~1/6 second) is a single shot from one person. More shooting = more freezes and jerkiness. I am thinking maybe there is some problem with loading the weapon fire sound.