[wrench]

I'm sorry that you are stuck with a new ECU but we did try to get you to check the things that were free and cheap first.

You will most likely need to fashion up a couple of jumper leads to use the KX coil, by remember you're just trying to figure out what's wrong before spending money.

You said that it goes flat on the top of the rev range in each gear causing you to short shift, correct? This is what is leading me to the coil. You said that you were in a rocky section and hauling the mail before the problem started. Again, this leads me to the coil (or other electrical part) because of the high demand that you were placing on the system.

As cylinder temp and pressure rises, the demand on the ignition system also increases or there will not be any spark to ignite at the plug. When a tired coil gets hot it cannot meet this demand and the spark gets weak resulting in a loss of power.
It could be a corroded connection in the plug cap, a small crack or burn spot on the plug wire (HT lead) or a crack in the plug insulator (porcelain).

Now to throw another thought into the mix. Someone else mentioned stopped up vent hose on the carb. What about the air bleeds in the intake throat of the carb. Have you made sure that they were not full of oil from an over oiled filter? That gets overlooked a lot.