If you hold it at a nose-up and leave it there, what happens? It sounds like you went nose-up then back to level fairly quickly. Leave it nose up for a while and see what happens. And instead of a 45degree, try straight up. The engine should be able to pull fuel striaght up while at full throttle.

It does sound like what the other guys said, a problem inside the tank with the clunk line, either the clunk came off the line, or there's a hole in the clunk line, or the line is off the tank's tubing or something along those lines. My money is on a hole in the clunk line where it attaches to the tank's tubing. I've had this happen plenty of times myself, usually due to being sloppy putting the tank together.

The Alpha tanks are known for leaking. It seems that sometimes they over-tighten the stoppers at the factory, causing the tank to split.

Here's an idea if you don't feel like pulling the tank out right now. Go get another fuel tank and strap it to the side of the fuse and test run the engine using that external tank. If you can do a left test and whatnot with that tank, but not the one in the airplane, then you know where the problem is.