I followed instructions tuning up the engine, including the line pinch test, all seems well, engine idles well, and accelerates without stalling, I have it slightly rich, A3 glow plug, 15% nitro w/ 18% oil, engine has had about 10 fuel tanks through it, good compression, I just installed it on a newly built trainer.

The PROBLEM I HAVE IS WHEN I HOLD THE PLANE UPWARDS IN A VERTICAL POSITION, THE ENGINE STALLS. When I put it down to horizontal it works ok this happens in all cases, idle, and full throtle.

I do see some bubbles in the fuel line, I will remove the tank to inspect for any kinks / holes in the fuel line, can there be another reason for these kind of stall?
Does the lenght / rise of the fuel line over the carburator plays a difference? The tank is installed about 1/2" below the bottom of the carburator, it's the only way.

Thanks in advance for your inputs.

The centerline of the fuel tank should be through the center of the carburetor or no more than about 3/8" lower. Your tank sounds like it's much too low, which is part of your difficulty. You also may not be getting good pressure to the tank. Check to make sure there are no obstructions in the system.