[ngoma]

I was going to say clogged fuel filter because that was the cause of most of any sudden no-rev or low power problems I have encountered in the past, but they never spontaneously solved themselves like in your situation.

Is it temperature related?

Doubt adding 2-stroke oil to the tank will solve this for you.

The fuel shutoff solenoid rarely fails so try some easier things first.

Splice in a section of clear fuel hose in the return line from the IP to watch for air bubbles. That may give you a clue that air is entering the fuel supply, which can limit revs and power.