<
When he pulled the plugs again, the same two were fouled, despite the coil swap.>


Yes, that is why we always change the resistor plug connectors under the coils first.....and everytime the plugs are changed. One bad Coil Connector effects both plugs on that system b/c the plugs are wired in Series.
The problem is the connectors under the coil are a spring contact, whereas the other plug in that series has a much better snap connecton.
They are a maint. item and last about 50K. As they age , they burden the coils, with the main fear of taxing the trigger circuits in the ECU. Being a DIS system, all switching is done at ECU and to tax that can be very costly..so, the connectors under the coils w/plug change is cheap insurance.
Plus, a large percentage of mis-fires on DIS HFM systems is those 3 connectors. It is always the first recommend when misfire is the condition. If coils are suspect on that chassis , one simply goes into pin #8 of the Module and looks for a 21/22/23 codes ..those usually pop with a coil malfunction..it is after we see that , that we do the swap and then simply go back and see if , for example, if a 22 has moved to a 23 ..then we know right away the condition has followed the offending coil..no driving around for weeks and looking at plugs ,etc..

__________________
A Dalton