My 1983 W126 (280SEL) is also experiencing the same problems. I have 3 further questions to your post:
1. Does the adjustment of the exhaust valve and replacement of the 2 rocker shims constitute a head job?
2. What test did the mechanic do to find out which exhaust valve to be adjusted?
3. What plug wires did you get? Stock Beru wires?
I hope to get a similar diagnosis done by my mechanic soon, and I need your advice. Thanks a lot!