Loss of Power after Warm Up

[Mud Diver]

So, my friend has a GPW and he says it looses power after reaching operating temperature. My first thought was vapor lock or possibly a vacuum leak from a warped something from the heat. Anyone experience this?

 

[Adrok]

Anecdotal experience:
Once the engine warmed up, loss of power and felt like a misfire when accelerating / higher RPM.
Turned out to be spark plug wires. Heat and slight position change from start up, it began arching to the block.
Running the motor in the dark, arching is readily obvious.
New wires, sleeved the wires with heat shield, full resolution.

Absolutely check for air leaks, intake and vacuum lines.
Sounds like you have a solid plan of action.
Cheers