<HTML>Resons for loss of power.
Since the problem is a progressive one (ie as time passes while it is running) it could be a few things.
1.) Your piston and cylinder are worn. This would not be evident at the start of the engine, since everything is basicly not heated and expanded. However, as time passes things heat, and expand. The "oil" in the mixture would also keep the seal between rings piston and cylinder tight, until it heated up and of course thined with the heat...causing compression break down.
2.) Bad Spark Plug. Which when heated can loose power.
3.) Poor fuel to oil mixture. Make certian it's the correct mixture for the engine.
Hope it helps!!