Old brittle wire harness can be causing intermittent failures, bad or missing electrical grounds, bad engine computer/ECU, or connections/grounds. Should we assume they checked all the basic stuff like timing, fuel pressure, vacuum leaks? Are they scanning for live data while driving when the problem occurs? Obviously without having the same access to the car they have in their shop, it's hard to make suggestions on what to check. I will only say this, I once worked on a car chasing constant multiple random sensor failures, the problem was too many electrical grounds on the wire harness corroded or rotted away, causing a "floating ground" which masked many of the sensor triggers confusing the ECU. It required many hours of painstaking probing and rewiring to fix the problem. And problems like that are impossible to diagnose by a scan tool or without having physical access to car.
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