185 Evinrude semi-shutdown

[1988250coastal]

I have an older Evinrude 185 which will fire right up, runs perfect for 15-20 minutes or sometimes all day. I can be motoring along and then the motor slows and the boat falls of plane. It acts as if you throttle down without touching anything. It has been to the shop, has had mechanical fuel pump changed, tried electric fuel pump, carbs rebuilt, all new fuel lines, fuel tanks cleaned, new plugs, new packs, water pump, thermostats changed, pre mix on the fuel. Has a demon it it somewhere! Any ideas other than exorcisom or drowning would be greatly appreciated.

[boats.net]

Probably only way of finding problem is to test ignition and fuel system at the time it stalls otherwise everthing will test ok.