Don't have the answer, but I've seen pretty much every engine from a P-60 thru a P-200 stick at one time or another. The P-200 was bad enough that we carried a dowel to reach up the tailpipe and spin the motor to free it up. For me, its been mostly P-80's, but not so often lately. I was thinking it had something to do with Aeroshell 500 oil, but recently, I saw a P-60 stick and the owner was using JetCat oil.

If I had to guess, I would say the needle valve for the fuel feed to oil the bearings is set too rich and the bearings gum up. But then, I don't really know.