Not enough tank pressure due to the exhaust system not producing enough back pressure.
Had the exact same problem on an ST90 in a Super Skybolt. Ran great on the ground, died at a half tank like clockwork. Did all the same fuel system checks as mentioned, no joy. It had a Tatone Pitts style muffler with dual outlets. Blocked one of the outlets and it ran perfectly throughout the flight and never dead sticked on me again. I recommend you try restricting the outlet of the exhaust to increase pressure in the tank and try again. Do your tests at half tank, as that is where your problem occurs.