1978 85hp Evinrude power cutout

[Glsjr40]

A problem I've been living with the last few summers: typical scenario - start motor and warm for few minutes before taking off. Run at 2/3 to 3/4 throttle for one to five minutes and suddenly motor loses power - still running smoothly but not enough power to plane boat. I cut back throttle and run like that a short distance and start fishing. After a few minutes I start up and move a short distance (with restricted power) and stop and fish awhile. I may repeat the sequence over half an hour to one hour or more. Eventually at some point I'll start up and the motor will run at full and normal power, and usually I won't experience any malfunction the rest of the day. Occasionally when running at low power and lowered throttle, the motor will suddenly grab power and run normally. I've replaced all fuel lines and hoses and rebuilt the carb's. In the real world of probability, is this most likely fuel system related or is it ignition system related?

[boats.net]

May be loosing fire to one cylinder, check each cylinder with timing light to see it drops a cylinder.