I own 2001 Kia Optima LX, it has 121k miles on it. The check engine light is on, and recently it jerks when moving. It does not pickup speed. It moves at 20-40 miles/hour, then after a while it picks up speed. sometimes it just turns off and locks the steering wheel on the spot. The car is a manual transmission. Any idea of what could be the problem.

The locking steering wheel is a mystery to me , but the poor performance and check engine light make me think fuel filter / fuel pump.
I had a very bad experience with an 02 kia , only had 50,000 miles on it when the timing belt went and destroyed the engine , so........ i always mention to people with kia's to have the belt adjusted and inspected or replaced if you don't know how old the belt is.