99Sonata

I just bought a 1999 2.4L, 4 cyl. Hyundai Sonata.
Occassionally I step on the gas and the car will barely move, then after a
few seconds it will start to go. This seems to happen only within the first
15 minutes of starting the car and when I am taking off from a complete
stop.
I have changed the plugs, wires, head gasket, transmission fluid&filter,
oil fluid&filter,timing belt&crankcase sensor.
The Engine light is on (solid). The engine code is Error Code:171 Left
Bank Lean.
Any ideas?
Thanks!