Fiesta 1.25 Keeps Stalling When Pulling Away

I've not long bought a Mk6 Fiesta with a dinky 1.25 engine. The last owner had a new Lander sensor fitted when he sold it, and the fuel economy is just excellent. I've had a new clutch fitted as the old one had broken up after 68k miles. There are two problems though that still plague the car. One is a heavy knock as you pull away, which is low down, and sometimes the steering pulls to the right, including during breaking. Secondly, the engine abruptly stalls as your driving away, including down hill. One time it stalled, the engine refused to restart for quite a while, despite lots of cranking. Last night I reved the engine a little while waiting at the lights, only to have the engine management light flash when it returned and dipped below idle.

The engine doesn't suffer the hunting problem, and keeps 800rpm when sat at idle, but having driven a few ford's, this one feels very flat and lazy through many of the driving ranges, mainly at the lower speeds. It pulls like a tractor, but it lacks the power to accelerate at what I would consider a good rate, even for a small engine.