Sounds quite similar to a problem I encountered with my profile fun fly some time back which originally had the tank buried in the wing. The engine would run great in any horizontal position but as soon as the nose was raised it died. The only way it would run with the nose raised was to set the H/S needle very rich which cut the power significantly.

The folks here were more than helpful in assisting in trouble shooting my problem. After numerous suggestions and as many trials on my part, nothing worked. Then someone suggested I try a test tank closer to the engine, as it was a sure thing that I had a fuel draw problem. l rubber banded on a test tank to the fuselage closer to the engine and presto, my problem was solved .

It turned out that even thought the engine ran beautifully for a short while at first, the clunk supplying fuel was just too far away from the engine when the nose was raised causing the engine to go lean and die. The plane did look quite sleek with the tank hidden in the wing, but it sure fly great with it mounted on the side of the fuse. HTH