A black top of the piston usually means that the engine runs on the rich side. When it gets too hot it usually burns those deposits off and becomes gray-ish or even looks like it's been sandblasted.

As I wrote in my "Help" thread, it's a complete mistery to me.

One thing that I could imagine, is that the sleeve expands soo much when it's getting to temperature, that the compression goes away.

Mine is making horsepower like crazy until it reaches 85-90 degrees, and then it has the same problems like yours.