2001 Infiniti I30 Engine stops at low RPM

Since I replaced the Catalytic Converter Bay2, the engine was responding slow. It felt like to me either the fuel injector or fuel pump is not working right.
After few months my car engine stops at low RPM. At stop light I had to press the break and the Gas Paddle (using two feet) a little bit so the can does not stop. When the car starts moving then the problem is gone.

I ususlly work on my CAR and I adjusted the engine idle speed by tightning the pair of cables. (One connected to the gas paddle and other I believe connected to a vacuam unit under the hood.

Engine does not stop at idle speed when I am driving but when I start the engine I have to press the gas paddle a little for the start and keep it pressed for 30 seconds to keep the engine running.

Though it temporarily solved my problem, I found a engine error code "P0505", "IAC circuit Condition or Idle Speed Higher/Lower than expected".

Could you suggest me what is my problem? Is it a Mass Air Flow Unit or fuel Injector or even Fuel Pump? Also I would like to know how the parts look like and where (in the engine) it is attached so I can find by myself and repair it. Suggest me online stores where I can buy parts cheaper.