Low power

Hello, I have recently bought the car listed above RS model used. It started having a problem with the rpm's jumping down anywhere from 500 rpm to 2,000 rpm depending on how much I press the gas pedal past a certain point as long as I only press the gas far enough to hold about 35-40 mph on a straight away it's fine but any farther or going uphill with the pedal pressed to maintain any more than about 20 mph the rpm's drop. There's also a white smoke coming from under the car where what seems to be near the start of the exhaust system. I was wondering if you know what the problem is and/or a way to fix it? Please and thank you.