2005 Softtail dropping one cylinder

[xcavater]

While on a ride with a friend his softtail began running on only the rear cylinder. We pulled to the side and I checked the plugs, plug wires, and all connections. Found no spark coming out of one side of the coil.
He mentioned that this happened a couple of times previously and thought it was the plugs. He would change the plugs out and it would be normal again.
I said that a coil could present this problem when hot and in the time it took to change out the plugs (he doesn't carry spares so we're talking 90 minutes plus) it cooled down and thus the problem disappeared.
On our discussion he went ahead and bought a coil and the problem still occurs.
Now he wants to replace the ECM!
He lives about 75 miles away so I've tried to help him remotely, problem is there is another who is leading him down the ECM route.
Can anyone give some troubleshooting points that I can relay?

 

[HDDon]

Before he buys anything try spraying the coil with electronic cooling spray to see if it is the coil. It could also be a plug wire breaking down under heat.

 

[xcavater]

Before he buys anything try spraying the coil with electronic cooling spray to see if it is the coil. It could also be a plug wire breaking down under heat.

I check the output from the coil on the side of the road. Definitely no spark out of one side. He replaced the coil after I mentioned it could happen when hot. Still acting up so its before the coil.

 

[HDDon]

Have you/he checked all the connections before the coil?

 

[xcavater]

Have you/he checked all the connections before the coil?

Not since the side of the road. I gave it a quick look/wiggle test didn't find anything; even r&r'ed the coil. By that time it had straightened itself out. That's what led to the discussion of heat and coil problems.