Usually an engine that just "shuts off" at WOT is running into one of two things. . The ignition is overheating and just shutting down, and will restart normally once cooled off (land and it flips and fires fine. . very frustrating) OR that the carburetor is not able to draw gas for some reason and basically puts a vacuum on the fuel line, inducing vapor lock and shut down. The fuel delivery problem will show signs of hard starting, necessitating choke and numerous flips to restart, and then will run fine until enough heat builds up at full throttle to again vapor-lock the gas in the line.

I'd chase the fuel delivery problem first, if I was you, checking all the lines, and for debris in the tank. I'd also shorten the line as much as possible, zip-tie the ends to the nipple on the carburetor and tank, AND route it as far from heat (even wrap it with insulating foam if necessary) to prevent this phenomenon.

The ignition problem, if it is occuring, CAN be caused by a questionable switch contact going intermittent from heat as the ignition demands more current at high power settings, the same for a bad connector anywhere in the circuit, a weak cell in the ignition battery pack which will have the same current-limiting effect under higher amperage loads. I'd advise checking every connector and the switch, do a 3-4 amp load test on the wiring to check for a voltage/current drop among the the components, and advise going to a 5-cell pack with a 6 volt regulator.

There's no reason TWO engines should experience the same exact problems under heavy loads, and the issues above are the most likely to have occured.

Best of luck with the little beastie.