It might be a heat problem. If it'll run for 8-10 minutes then die you might be getting a bit of vapor-lock in the fuel line.

Is the fuel line routed close to the exhaust? I was having a similar problem, but it was never enough to kill the engine (unless it was over 100 degrees out).

Testing a theory, I ran a water hose on the fuel line and mechanical pump. The clear filter never ran low, but if I turned off the water stream it would. Once enough heat built up the engine would start to stumble.