Another common problem that causes engine performance issues is vacuum leaks. Vacuum leaks are air leaks into the engine that occur after the throttle body and before the intake valves. Any leaks in this area are considered vacuum leaks. Vacuum leaks upset the air/fuel mixture and can cause misfires, a loss of performance, idle issues, or poor fuel mileage. Vacuum leaks are on the top of my list of probable causes for performance issues. The engine likes balance; anything that upsets that balance will cause performance issues. Vacuum leaks are a very common cause of this imbalance.
How do you go about determining if you have a vacuum leak, and how do you determine its location? Some methods are quick and easy, while others are a bit more involved and require special tools. I like easy, so I normally grab a can of carburetor cleaner and spray around the suspected areas while the engine is running to see if there is a change in RPM. If I find an area where I spray and the RPM changes or I can actually hear a gurgling sound as I hit the source of the leak, I know I've found a vacuum leak and I've got a pretty good idea where my leak is.
I will admit that doing this to a hot engine can get dangerous, as carburetor cleaner is flammable. No worries, because you can also use water in a spray bottle to do pretty much the same thing. It's not as accurate, but it does get the job done.
One other method is to use smoke. While the engine is off, you can introduce smoke into the intake through one of the vacuum lines and look for where the smoke comes out. If you see smoke coming out, then you've found the source of your leak. There are machines that produce smoke, but I've seen people do pretty much the same thing using smoke from a cigar.
The method you choose is up to you, but I would suggest including checking for vacuum leaks in your diagnosis. Here's a video on finding vacuum leaks you may find useful.