If no gas smell in oil, FP diaphragm is good.

Water in fuel will sit in the bottom of the tank, and highway driving will let the puddle settle near pickup. Try a can of fuel drier or a bottle of rubbing alcohol in the tank.

Remove air filter lid and look down carb throat with engine idling. See any fuel droplets vaporizing on the throttle plate (choke must be open, engine warmed up). If so, carb needle valve is leaking and float bowl is flooding into carb, making an extremely rich mixture. Downshifting puts engine in WOT mode, which needs a richer mix. Upshift puts you back in lean cruise, and the rich mixture causes power loss.