augy

A valve adjustment issue could only be causing misfiring in the case of MASSIVE seperation between the cams and rocker arms, and he would definately be hearing the sound of cam smacking rocker arm. Given that he knew how to test the fuel injectors, I'm going to guess that he would know that the clacking means it's time for a valve adjustment. He could have them too tight and be experiencing valve float, but he would hear that just as well.
If you meant valve JOB, that is in fact a possibility, however he would definately be experiencing more problems during idle if the valve seats were so deformed that he's experiencing the kind of problems he's described.