There are many reasons an engine misfires. It is best to start with basic things and move on to more complex things. With 134000 miles you should start with the spark plugs and wires. Pull the plugs and look at them. If the gaps are too large and the electrode is worn down replace them. If not, replacing them will not hurt, but it won't solve the problem. Next test the resistance of the spark plug wires. If they are out of spec replace them. If you are going to do the work yourself it is best to purchase a repair manual for your car. Even if you have a mechanic do the work it may be helpfull to purchase a manual. A repair manual can tell you some information specific to your car and give you an idea on what a mechanic may say is wrong with your car.
Either way we will help you with this problem. Start with the spark plugs and we will take it from there.
BTW--performing basic repairs is not difficult. Just some basic tools, a manual, and a little elbow grease can save you big bucks.