When your car feels rough and bouncy when the engine is running, there are several reasons why this happens - some major and some minor. The best course of action is to take your vehicle to a certified auto service center, but there are a few things it could be.
A common and easy cause to fix is a loose or broken vacuum hose. They can wear easily over time, but by looking in the engine compartment you should be able to see if there are signs of abrasion, breaking or loosening. Modern cars use sophisticated fuel injection systems to meter fuel, which relies on a vacuum to help the engine's computer determine the precise engine condition. If not working properly it can cause serious engine damage.

Clogged filters can also cause rough idling. It's hard to tell if a filter is clogged, so it's best to replace at each oil change. An air filter cleans the air entering the engine and prevents debris from entering the engine, causing damage. As the capacity to filter the air going into the engine is reduced, the engine is not able to function properly.

There could also be something wrong with the vehicles' electrical components. Bad spark plugs, bad spark plug wires, and a bad distributor cap are other common causes of rough idling. These items are all important parts of what keeps a vehicle running. Spark plugs provide the spark that ignites the air and fuel mixture within the cylinders. This continuous ignition is what keeps a car moving down the road. A distributor cap regulates the firing order of your vehicle's engine.

These are just a few items that could cause a rough idle. It's best to take a vehicle in for regular maintenance with an certified service center to keep it in good working order.