Is the jittering hardware related? It may sound patronising to ask but gotta cover all bases.
To check, is the screen sort of 'locking up'? What game?
If it's a Source engine game, enable the developer console and then in said console enter net_graph 1. If there are lots of ping spikes then it's network related, if not and it seems more stuttery or like it's locking up, try turning details in the game down.
Do you use VOIP or any similar things? Streaming audio? Does that spike? What game server?
I know it's a lot of information to gather but as I said, cover all bases and you'll get all the problems sorted
