My Thunder Tiger Pro .61 did the same thing today at the field. I was trying to get it to idle reliably for the maiden flight of my 4*60. I'm running 15% nitro and had a McCoy idle bar type glow plug. What you described to a tee: It would Idle for about 8 seconds and then die. It acted like the low needle was too rich, but whenever I'd lean it a tiny bit, it wouldn't start! My instructor and I kept playing with it until we finally got running OK, but not good by any means. I don't see anything obvious with the fuel system at this point. The tank seems to be holding pressure fine.