It's hard to say why he must have been trying to fix it or find issues I'd guess.

The thing is unless you know if it's fuel or electrical you can try chasing faults forever. An afr plot is the first key, carefully watch the afr just before it misfires as this Can tell you if it's heading rich or lean into misfire teratory.
The aft during the miss will be irratic, but just before will give you a good indication of where it's at.

As example if it's running 24psi and the afr is flat 12:1 it's not going to miss there because of a fuel related issue. So you could start looking at ign, ecu sensors, loom, sensor gaps etc but until you have some data your pissing in the wind