Maybe the cylinders are a little to small so when the pistons get hot it seizes up. My first stroker had that problem. The stroker kit I bought came with blower pistons and the machine shop set the engine up for regular pistons. When the engine got warm it would seize up. It required a complete rebuild to fix it. All it takes is one small mistake when you build an engine. My advice is to pull the engine out and have a good mechanic go thru it. Might be an easy fix once it is all apart.