I had similar symptoms on my old welcraft both engines developed a similar fault.
once lost power instantly then all off a sudden power was back, then it was gone. it was all or nothing and intermittent.
The fault was the big multi wire adaptor (round rubber coated plug) with multiple round pins for connection. Now, i'd cleaned this previously and vaselined it and all looked fine, but pushed a screw driver in between the pins to widen them all about 1mm, problem went away.
So if the power problem is an all or nothing type thing check electrics.
The problem with the other engine was;
the engine seemed to slow up and lose power gradually, almost seemed as if I was easing back on the throttle.
Turned out the coil was bad, when it heated up it broke down.
hope that helps