Spring Flying - ARF Streak

Last Wednesday a bunch of us went flying at the club field. The weather was sunny and in the 80's. And early in the afternoon there wasn't much wind.
Remember the ARF Streak? Well, my first flight of the day was with the ARF Streak. It performed well for about a third of a tank, and then the engine began sagging. And then it quit prematurely. One of the guys suggested the needle valve assembly had some dirt in it. But I just wanted to try again. So I restarted and tried to fly with the needle valve opened up a lot compared to the first flight. But the engine just sagged and quit.
So that was it for the Streak.
Since then, I have flushed out the tank with denatured alcohol. And removed the NVA and blown it out and reinstalled it. It did seem that there was some dirt on the needle when I took it out of the spraybar, but otherwise I could not see anything blocking anything.
I also added an inline fuel filter to the fuel line between the tank and the engine.
Hopefully the engine (OS LA25) will run O.K. the next time I try to fly it. Where do these gremlins come from anyway?