While driving felt a jolt and a bit of smoke coming out from under the bonnet

Hi, my classic Corolla has been working very well lately, but last week, after about thirty minutes of driving in fairly warm conditions, I felt a little 'jolt', after which I noticed a bit of smoke coming out from under the bonnet. I also felt a vibration through the steering wheel yet was on a very smooth road, and the gears felt a bit 'buttery', as if they weren't really grabbing/driving the engine. She was still going okay, but was reluctant to accelerate. There was a strange smell coming from the opening at the bottom of the gear-stick, which was also present once I stopped and lifted the bonnet, as well as under the cavity of the driver's side wheel. She hadn't overheated (indicator was on 9 o'clock). The emergency roadside assistance added water to the radiator, but said she wasn't too dry at all. She was idling fine, so I took her for a little drive around the carpark where I had stopped, but I felt the gears were a bit strange, and didn't want to drive her back home in heavy traffic. I had her towed to her mechanic. Could this be a gearbox issue, or perhaps she lost power to one of the cylinders? So far the mechanic can't find fault with her, so I thought I'd ask for a second. Opinion. You've been helpful to me in the past, so thank-you in advance for your invaluable assistance, once again. :)