its just that way sometimes, probably due to the limitations of the engine
eg. take a look at my rig, and then pull your hair out because of the choppiness of running the game carmageddon 3 from 2001 - its mostly smooth, but theres moments of insanity where the fps dips for no reason...
sharpbricks got about all the advice you need to get that second monitor running, you can always search for the driver for the monitor, but i doubt it'll be floating out on the net to be easily found (though this most likely wont fix your problem)
if your trying to run the mini dp connection, theres the whole active v passive fiasco you can google...
good luck with your setup!