Weak Fuel pump. Had the exact same thing happening on a brand new P-100. I set the Idle RPM up 500 and it would not flame out. However, the plane would move on its own at idle. When I called George to ask what was happening, he sent me a new fuel pump. I set the RPM back to the OEM set point and put in the new fuel pump. I have about 50 flights on it now w/o a flame out.

It is kind of a bummer doing a stall turn and the flame goes out when you pull the throttle all the way back. It would flame out on the runway as I landed too. For me, ... setting up the idle worked for a temporary bandaid fix to get me thru the event. Setting up the throttle on the SPT5 could be a bit harry if it is like the one we have in the club here. That plane is already a handful to land. Can't imagine landing it with the prop spinning more RPM at idle.