There could be a vacuum leak from a vacuum hose, a gasket, or a component that works with vacuum (for example, EGR valve). However, there are different types of faults that can cause engine surge (engine speeding up and down while holding the accelerator steady): vacuum leak, worn fuel pump, troubles with a fuel pressure regulator, faulty PCV valve, EGR valve stuck, and faulty sensor(s) (in emission control systems, mainly). First, download diagnostic trouble codes, even if you don't see the check engine light on. If there are some systems in need of maintenance (spark plugs, wires, fuel filter, air filter) check those, you'll need those items either way. Check for good fuel pressure, if necessary.