The problem could be a bad spark plug or a bad fuel injector. If changing out the spark plugs (tune up) does not fix it, then have the fuel injectors checked out. If not enough fuel is released then it will cause the engine to misfire.
This is just part of a car getting older and is nothing serious. you can either attempt to replace the exhaust manifold, or to simply replace the engine, both are costly and unnecessary repairs.
Do a check of the engine looking for broken or loose ignition components. Check the computer to see if it has any error codes. Try doing a tune up also.
I would replace the distributor along with the spark plugs and wires. This would address all of the components of the ignition system. If the problem still remains after all of these replacements I would take the car to the dealer. You can also performa compression test if the problem still exists.
first open the all sparking plugs and put them a pane and put some petrol in it and wash them in petrol and rub with piece of ambrey paper and put them back in to cylinders,this process will fix the misfire problem.