shots from the garage...

I think you got a good start by checking the valve springs..And a compression test... After that "Myself" would look at ignition..And then Fuel system flush and pressure/flow check?.. Sounds like the fuel adjustment was just trying to cover something up?
Also Not sure... But just food for thought.... Check or install a good ground strap between the body and motor...
I just found this problem on a buddy's car...lol Back in the 9,30's... Now i'm a genus's ! Ha.

Go test "Bill Cosby Style" Any side street...lol Just Joking you know !!!!