Engine vibrates a lot at idle.

My truck (a nissan 720) has the 2.4l Z24 engine with carburetor and lately it idols at around 1200 RPM, which higher than it should I believe, and it intermittently vibrates fairly intensely, enough to shake the stick back and forth and make things shake inside the cabin. It does this every 10 or 15 seconds or so. And if I rev it up then sometimes it drops way low to like 200 RPM and almost dies if I don't give it some gas. I just changed the spark plugs, oil, and air filter, I replaced the OEM filter with a K&N. What might the problem be? Any help is appreciated. Thanks.