I agree with the last post. I ran three tanks on the ground as well (with the engine on the plane, cowl off, and with the plane tied down) and the OS 1.6 has run flawlessly ever since (20+ flights). Often times when a plane stalls in flight prematurely, it could be a fuel delivery problem. Check your fuel tank (especially the clunk), lines, and connections. Also be sure the middle of the tank is somewhat near the level of the needle valve. IMHO, you shouldn't fly the plane until you get the engine running with a high level of confidence as one gets so worried about the thing quitting, its hard to enjoy the flight Is there a particular time in the flight or attitude the that engine usually quits?

I had an OS 40 on my first trainer that had the same problem. I replaced the standard tank with a Hayes, replaced the fuel lines, raised the tank, and used hose clamps - never had a problem again.