Kia opirus

I got a Toyota Camry 1999 model with a confirmed P0340 and a pending P0116, P0773 code. The symptoms the customer told me were: engine stalling while on motion and during that time the car just wouldn't start because the engine was very hot but when the waited for an hour or more the car later started. What I noticed was that the car revv high intermittently on it's own but will return to normal idle again about 5 second again, and the exhaust tailpipe was really darkened when I put my hand. Plugs, fuel pump and a relay( the don't know where the relay was removed though) was replaced by another engineer but the problem still persist. Please how do I fix this problem?
Will replacement of part like camshaft position sensor fix this problem?

Thank you so much for your prompt response and assistance to help me fix the car problem.
I checked there and didn't see the CMP sensor nor a hole where it should've fitted into however I saw the connector only. Fortunately, I saw the fault today. I was imagining why the code for temperature sensor displayed hence it prompted me to ask the car owner if they've ever experienced overheating with the car. He acknowledged that fact and was more detailed in his explanation, that the fan was not working and radiator have been leaking and the keep adding water. It was the cooling system that was causing the water temperature sensor code to show. The radiator was leaking water and one among the two fan was not working so when the car is hot, it stalls. But it will restart after the car is left to cool. Although I didn't test run it because he told me the fuel was low.
I have fixed the fan. Both are working now but for the radiator I told them to give it to a specialist on radiator to fix the leak. I'm hoping that if they fix the radiator the problem will be permanently fixed.
In addition, I have noticed that the timing belt condition needs to be replaced soon because it was getting aged.

The camshaft position sensor doesn't even seem to be removed by someone from the engine. It's seems the car doesn't come with a CMP sensor but CKP sensor only.
It could also be because of the timing belt that was wearing out that caused the CMP sensor code.