Rough idle

Car idles rough, when driving runs better, engine shakes when idling, revs drops and threatens to stall. I have replaced air intake box and filter plus the hose. Cleaned the MAF sensor and throttle body recently which did not help. Spark plugs replaced a few months ago by mechanic friend. It does not feel like it is not running on all cylinders. I want to sell this car for approx $1,200.00 so do not want to spend a lot of money on it, but just need to get the idling right. It it has dual fuel but have not been running it. Thanks