if you aren't using v1.8i... please update. there was a bug with v1.8h (which was only 'out' for 3 days before being replaced) that would increase the load on the system if you were using modifiers.

other then that, i do not know. i have zero problems playing hours on end with very low end equipment (i do get a mouse glitch once in a while.. that is being worked on)

as for your machine staggering, do you have any anti-virus software running while you're playing? you say your 3 are running at 800x600... is that in 1600x1200 screen res? and your main is 1680 x 1050? try reducing the screen res and see if that helps.

also, do you run at low to medium in-game details?