The most common cause of acceleration issues lies in the transmission. The transmission converts the mechanical energy of the engine and transports it to the wheels, thus causing the vehicle to accelerate. Transmissions are expensive to repair and are even more costly to buy new.
A burning smell coming from under the vehicle is an indicator of a bad transmission, as is a grinding sound while driving. Getting an experienced mechanic to examine the transmission will enable you to determine exactly what is wrong with the transmission. Sometimes transmission issues are easily fixed, so finding a trustworthy mechanic is a must.