Easy enough. Once it warms up take the air cleaner lid off and see if the choke is open. Or start it up cold and see if the choke opens as it warms up. It could be the accelerator pump. This damned ethanol gas eats up rubber like Ms PacMan. Once things get warm and expand, the pump diaphragm could be letting gas slide by it, causing the stumble. On the electrical side, I've seen coils break down when warmed up and the symptoms are similiar. Hard to pin down, but when in doubt revert to the basics. It's likely something very simple. Good luck and hope this helps.
