Camshaft sensor?

I brought my car listed above turbo to mechanic said needed new camshaft sensor and timing done. He did that and was running great for a while but after a few months, back to the same problem. Brought it back and they said the camshaft sensor they put in was no good so put in another. Again, it was running good for a while. Now back to the same problem. Shouldn't he have changed the timing chain along with doing the timing? (The timing chain is known to have problems on this model) Just ordered a new camshaft sensor from VW. How hard is it to replace? Help