Vw polo 2000 1.4 16v Rough idle

[Sof]

I recently gave my car for cylinder gasket replacement. After the replacement the mechanic has informed me that he needed to change the camshaft sensor so it was replaced.

After I take out the car from the work shop I had a difficult time driving it as the car is very rough when the engine warms up it cuts the engine and the idel goes up and down between rpm 1.2 and. 9 continuesly which makes it very hard to drive.
I gave back the car to the same work shop and now it has been 4 weeks and they were not able to figure out the issue.
I did little bit of my own investigation and I got P0342 fault the from the scan tool.

Any help will be appreciated.