2000 Opel Astra Performance

My 2000 Opel Astra 1.6 mileage 152000KM (95000 miles) car is not going beyond 80KM/hr (50 miles)speed on freeways with full throttle. It also jerks at lower gear when the air condition is on. It doesn't jerk if I off the Air condition.I took it to the dealer. They said that engine doesn't have any issues and changed all filters(fuel, air, oil etc). But the problem is not resolved. One of local car mechanic says to try changing the spark plug sets. Car goes to up to 80km without any difficulties. Will it work correctly if I change the spark plugs? If not what could be the reason for this issue? Pls suggest me.