Yesterday I noticed that my car started bucking whenever I did a full acceleration, and soon it started bucking heavily all the time. Now it's impossible to drive it, implying you can even start it because of poor idle.
Sometimes I can drive half a mile normally and then it just starts bucking like hell again and sets off the SES light.

Diagnostics say it's error code 44 - Lean exhaust.

I did a lot of work on the engine last winter which means it could be a million different things. But it did run properly for months.
A month ago I started noticing bad idle every now and then but thought it was probably a small vacuum leak until yesterday. (There's a small gap between the air cleaner and the MAF sensor and I've spent hours trying to assemble the damn thing and there's always a gap.)
Also, the O2 sensor wire had to be lengthened due to new headers but if this was the problem, why did it work just fine for months?

Today I heard that the use of oiled K&N filters can break your MAF:
http://forums.automobilemag.com/70/6...you-thousands/
This is the first time I have used a K&N filter so I can't rule this out either.

 

All cyl firing?

 

Yeah. I've experienced only 7 cylinders firing and it didn't feel this rough.

I'm currently replacing the O2 sensor as I can't think of anything else where to start.

 

Replacing the O2 sensor seems to have helped with the bucking but the engine still pretty much dies every time I stop for traffic lights.
Unless I put it in parking gear.
Why is this?

 

Did you check the basics?

Check base timing, perhaps distributor came loose?
Do a good fuel system test, note pressures.

If a sensor was faulty IE reading values outside its specified range, the computer would pick it up and tell you via SES light or better yet, datalogging.

I would pay attention to the fact that it ran fine for months and got progressively worse.