Check your air filter after a run where you've lost your top speed. Is it soaked in gas? Sometimes carburetors kick a little fuel out onto the air filter. As the filter gets soaked in gas, it begins to block the air flow into the engine enriching the fuel/air mix and reducing power. After the moped has sat for a while, the gas evaporates, and everything goes back to normal.
Does your engine start to four-cycle (i.e. drop in pitch and lose power at top speed)? That's a pretty common symptom if this is your problem.