First thing that should have been done is checked the fuel pressure with a fuel pressure tester.
Replacing knock sensor was a waste of time and money. (Knock sensor very rarely fail, the code means that some other problem set the knock sensor code).
Also check the distributor for a bad bearing which will create a brownish dust on the optical sensor causing drivability problems. If bearing is bad the distributor wiil need to be replaced.
Make sure that your mechanic does NOT overtighten any of the drive belts (PS, ALT, A/C) as overtightening with cause stress on the crankshaft and break the snout off.
Let us now what he finds after checking the above.