Re: Loss of power
If the pump pressure drops during load, it can still supply enough fuel to keep some fuel in the bowl, but the level would be low.
A low fuel level causes it to go lean, robbing power.
It's a matter of how much it's not getting. Yes, if the bowl goes dry, it sputters and stumbles. It's a matter of being "in between" good supply and starvation.
Plugs can also be the culprit -- overheating plugs don't always create a misfire like a thump thump. They just don't ignite the mixture well, losing power. Slow down a bit, power comes back. Have a look at them. Bosch, Splitfires, Multis, and Plats have a reputation for doing what you describe. Use the plugs the engineers designed the combustion chambers around - good old fashioned spark plugs, not advertising scams.
And as suggested - check the timing advance.