Toyota Repair: toyota misfire, spark plug wires, distributor cap

Question
Great service ted.
I have a 99 1.6 manual,petrol avensis with 180,000 miles whitch I use as a Taxi.....recently the engine <spark plugs> started missing a little just after nearly every gear change....you could stop it a bit if you kept the revs hi....I replaced the plugs and she was perfect again but only for about a 1000 miles....back to the same misfiring.....allways after a gear change driving...
any ideas...thanks

Answer
Have the spark plug wires also been replaced, not sure what engine this is, does it have a distributor with the wires coming out of it or does it have no distributor with only coils? If it has plug wires try replacing them as well as the distributor cap and rotor if i has one.