98 Toyota Lucida

The Toyota Lucida, year 1998, engine 2430,petrol engine and model No. 2000 has been jerking and misfiring at varied speeds, at time it might jerk at 60 km/h, 80 or even 100 km/h. When it jerks the pulling also affected. We have changed the plugs, oil and the filters. It is still behaving the same. what could be the problem