There is air being sucked through the system from somewhere. Make sure your tubes that go through the Y festo are cut at 90 degrees flat. Check the clunk when there is 1/3 fuel left in the tanks to make sure its all the way on the bottom of the tanks. Are the tanks flat on the bottom or are they curved like a basin? If they are flat it's possible when there is 1/3 file left inside them and you are landing with nose slightly down attitude then most of that fuel is flowing forward into the tank and some air is being pulled into the UAT. This happened to my Ultra Flash tank. The fix was I tilted the tank 5mm or so down at the back of the tank where the clunk was and that fixed the problem. You can test this theory by flying half the time you normally fly and land where you have 1/2 tanks full. If the UAT is full then its clunks not seeing enough fuel even when 1/3 fuel. If this is not the problem then it's sucking air from else where. Also check your fill line on the UAT. It may be the plug on the fill line you use.