Sparks

Recently my car started seems like its misfiring and loss of power and having to hit the throttle more. I replaced my spark plugs and wires and the problem is still there. But when now when Im driving down the road, all of a sudden its like it just kicks in and runs like a dream for a bit, then goes back to misfiring. It has been cutting in and out like that on warmer days, but wont run good at all on really cold days. Does anyone know what and why its doing this? please help its killing me on gas!!