We've had the same last year and this year with our Suzuki 2hp 2 stroke tender engine! (Owned by us since new with minimal usage)
Similar symptoms last year in Salcombe and we found a rusty tank cap contaminating the fuel. Tank drained and cleaned, carb cleaned, new tank cap(£20 I think) and it worked fine all year.
Arriving in the Isles of Scilly this year, same symptoms again...! I repeated the above actions, there was a bit of muck in the carb, but still didn't like running at high speed. With the help of the mechanic on Bryher island we determined the fuel tap was the problem, and not letting enough fuel through. He said it was fairly common for these taps to age and fail as the rubber inside wears out. A scavenged secondhand one of those later, and the engine's been running happily the last few months now!