1999 Suzuki Swift Problems

My car still jumps and juddes when i reach about 65 to 70 mph. I had to slow back down to 30 at one piont on the dual carriege way to stop it from Juddin so much. The car doesnt have as much power in as it did before. Am bot sure whats wron...

I was doing 60 down the motor way trying to get up to 70 and the car was struggling, my gear box has recently gone, which is in process of being fixed, am not sure if that effects it or not, but it was jumping abot and struggling to go fast...