Jerking backfiring Toyota Corolla

Toyota corolla 1989 400, 000 k's 1.6 l sedan. When I drive the car for a little while it will heat up and then the problem occurs when I am accelerating with the pedal right down at first it will buck and jerk a little and loose a lot of power and then later on it does it worse and worse and it feels almost like a misfire. I have platinum spark plugs in could it be those ive heard that they are not very good for imports I have cleaned the egr valve the vacuum, I cleaned the fuel pump I have checked the whole ignition system.