My 87 Evinrude 60 goes into SLOW mode unexpectedly for no apparent reason. VRO has been bypassed so I'm running 50:1 in the tank. Motor starts easily and idles well. Leaves the marina smartly and runs up to 5000RPM just fine. At some point in the trip, the motor will cut back to 2200 RPM or so at WOT and the engine bogs until the throttle is pulled back. The motor then runs smoothly at 2200 RPM back to the marina. Pushing the primer switch causes the motor to bog. No alarms or steam. Fuel line bulb is firm. New plugs, wires, power pack and thermostat. Carbs are clean and the motor acts the same on a portable tank and hose as with the internal tank. Local mechanics are baffled. I've checked, cleaned and tightened all electrical connections and have found no frayed wires. We have spent over 2 hours at cruise (4200RPM) with no problems, other times it'll cut back - doesn't seem to be any common connection other than higher power levels. Some forums suggest checking the blocking diode, but I'm not having much luck finding it on this motor. Anybody have a thought?